Document individual signals (only the most common signals):
description, default action, availability.

(cherry picked from commit 400e1dbcad93061f1f7ab4735202daaa5e731507)

PyConfig_InitPythonConfig() and PyConfig_InitIsolatedConfig() no
longer return PyStatus: they cannot fail anymore.
(cherry picked from commit 8462a4936b3a551dc546a6adea04a70b0a07ca67)

Co-authored-by: Victor Stinner <vstinner@redhat.com>

For now, we'll rely on the fact that the config structures aren't covered by the stable ABI.

We may revisit this in the future if we further explore the idea of offering a stable embedding API.

Add a new struct_size field to PyPreConfig and PyConfig structures to
allow to modify these structures in the future without breaking the
backward compatibility.

* Replace private _config_version field with public struct_size field
  in PyPreConfig and PyConfig.
* Public PyPreConfig_InitIsolatedConfig() and
  PyPreConfig_InitPythonConfig()
  return type becomes PyStatus, instead of void.
* Internal _PyConfig_InitCompatConfig(),
  _PyPreConfig_InitCompatConfig(), _PyPreConfig_InitFromConfig(),
  _PyPreConfig_InitFromPreConfig() return type becomes PyStatus,
  instead of void.
* Remove _Py_CONFIG_VERSION
* Update the Initialization Configuration documentation.

(cherry picked from commit 441b10cf2855955c86565f8d59e72c2efc0f0a57)

* bpo-38234: Py_SetPath() uses the program full path (GH-16357)

Py_SetPath() now sets sys.executable to the program full path
(Py_GetProgramFullPath()), rather than to the program name
(Py_GetProgramName()).

Fix also memory leaks in pathconfig_set_from_config().

(cherry picked from commit 1ce152a42eaa917d7763bce93f1e1ca72530d7ca)

* bpo-38234: Add tests for Python init path config (GH-16358)


(cherry picked from commit bb6bf7d342b4503a6227fd209fac934905b6a1aa)

* bpo-38234: test_embed: test pyvenv.cfg and pybuilddir.txt (GH-16366)

Add test_init_pybuilddir() and test_init_pyvenv_cfg() to test_embed
to test pyvenv.cfg and pybuilddir.txt configuration files.

Fix sysconfig._generate_posix_vars(): pybuilddir.txt uses UTF-8
encoding, not ASCII.

(cherry picked from commit 52ad33abbfb6637d74932617c7013bae0ccf6e32)

* bpo-38234: Cleanup getpath.c (GH-16367)

* search_for_prefix() directly calls reduce() if found is greater
  than 0.
* Add calculate_pybuilddir() subfunction.
* search_for_prefix(): add path string buffer for readability.
* Fix some error handling code paths: release resources on error.
* calculate_read_pyenv(): rename tmpbuffer to filename.
* test.pythoninfo now also logs windows.dll_path

(cherry picked from commit 221fd84703c545408bbb4a6e0b58459651331f5c)

* bpo-38234: Fix test_embed pathconfig tests (GH-16390)

bpo-38234: On macOS and FreeBSD, the temporary directory can be
symbolic link. For example, /tmp can be a symbolic link to /var/tmp.
Call realpath() to resolve all symbolic links.

(cherry picked from commit 00508a7407d7d300b487532e2271534b20e378a7)

* bpo-38234: Add test_init_setpath_config() to test_embed (GH-16402)

* Add test_embed.test_init_setpath_config(): test Py_SetPath()
  with PyConfig.
* test_init_setpath() and test_init_setpythonhome() no longer call
  Py_SetProgramName(), but use the default program name.
* _PyPathConfig: isolated, site_import  and base_executable
  fields are now only available on Windows.
* If executable is set explicitly in the configuration, ignore
  calculated base_executable: _PyConfig_InitPathConfig() copies
  executable to base_executable.
* Complete path config documentation.

(cherry picked from commit 8bf39b606ef7b02c0279a80789f3c4824b0da5e9)

* bpo-38234: Complete init config documentation (GH-16404)


(cherry picked from commit 88feaecd46a8f427e30ef7ad8cfcddfe392a2402)

* bpo-38234: Fix test_embed.test_init_setpath_config() on FreeBSD (GH-16406)

Explicitly preinitializes with a Python preconfiguration to avoid
Py_SetPath() implicit preinitialization with a compat
preconfiguration.

Fix also test_init_setpath() and test_init_setpythonhome() on macOS:
use self.test_exe as the executable (and base_executable), rather
than shutil.which('python3').

(cherry picked from commit 49d99f01e6e51acec5ca57a02e857f0796bc418b)

* bpo-38234: Py_Initialize() sets global path configuration (GH-16421)

* Py_InitializeFromConfig() now writes PyConfig path configuration to
  the global path configuration (_Py_path_config).
* Add test_embed.test_get_pathconfig().
* Fix typo in _PyWideStringList_Join().

(cherry picked from commit 12f2f177fc483723406d7917194e7f655a20631b)

Python now dumps path configuration if it fails to import the Python
codecs of the filesystem and stdio encodings.

(cherry picked from commit fcdb027234566c4d506d6d753c7d5638490fb088)

* bpo-38234: Remove _PyPathConfig.dll_path (GH-16307)

The DLL path is not computed from any user configuration and cannot
be configured by PyConfig. Instead, add a new _Py_dll_path global variable.

Remove _PyConfig_SetPathConfig(): replaced with _PyPathConfig_Init().

Py_Initialize() now longer sets the "global path configuration",
but only initialize _Py_dll_path.

(cherry picked from commit c422167749f92d4170203e996a2c619c818335ea)

* bpo-38234: Fix PyConfig_Read() when Py_SetPath() was called (GH-16298)

* If Py_SetPath() has been called, _PyConfig_InitPathConfig() now
  uses its value.
* Py_Initialize() now longer copies path configuration from PyConfig
  to the global path configuration (_Py_path_config).

(cherry picked from commit e267793aa4101b2771ed0e66aaff5743d23f59af)

* bpo-38070: _Py_DumpTraceback() writes <no Python frame> (GH-16244)

When a Python thread has no frame, _Py_DumpTraceback() and
_Py_DumpTracebackThreads() now write "<no Python frame>", rather than
writing nothing.

(cherry picked from commit 8fa3e1740b3f03ea65ddb68411c2238c5f98eec2)

* bpo-38070: Enhance _PyObject_Dump() (GH-16243)

_PyObject_Dump() now dumps the object address for freed objects and
objects with ob_type=NULL.

(cherry picked from commit b39afb78768418d9405c4b528c80fa968ccc974d)

* bpo-38070: Add _PyRuntimeState.preinitializing (GH-16245)

Add _PyRuntimeState.preinitializing field: set to 1 while
Py_PreInitialize() is running.

_PyRuntimeState: rename also pre_initialized field to preinitialized.

(cherry picked from commit d3b904144e86e2442961de6a7dccecbe133d5c6d)

* bpo-38070: Py_FatalError() logs runtime state (GH-16246)


(cherry picked from commit 1ce16fb0977283ae42a9f8917bbca5f44aa69324)

This reverts commit 396e0a8d9dc65453cb9d53500d0a620602656cfe.

This reverts commit 6a150bcaeb190d1731b38ab9c7a5d1a352847ddc.