Provide more clarity for Win32 calling conventions - now that we always
compile to __cdecl on Win32, we'll define that as the the libgit2
calling convention.  Also offer NTAPI (__stdcall) calling conventions
for things that need callbacks from NTAPI code (eg fiber-local storage).

Move leak check initialization into git_win32_leakcheck_global_init, and
call it on library initialization.

msvc crtdbg is a mouthfull that is not particularly indicative of what
it does.  Let's rename it to "win32 leakcheck".

Refactor "global" state

This change:

* Increases MY_ROW_LIMIT to 2M, since it has been failing in #5595's
  tests since it's _super_ close to the limit.
* Calls `git_repository_free()` on a `git_repository` that was being
  leaked only in Windows.
* Marks the global `git_repository` on `tests/repo/init.c` as `NULL`
  after being freed to make any accidental access more noisy.
* Uses `cl_assert_equal_i()` in `test_trace_windows_stacktrace__leaks`
  to make the test failures more actionable.
* Renames the globals in `tests/repo/init.c` so that they don't start
  with an underscore.

Provide a mechanism for system components to register for initialization
and shutdown of the libgit2 runtime.

We want to store a pointer to emulate `pthread_exit` on Windows.  Do
this within the threading infrastructure so that it could potentially be
re-used outside of the context of libgit2 itself.

Our "global initialization" has accumulated some debris over the years.
It was previously responsible for both running the various global
initializers (that set up various subsystems) _and_ setting up the
"global state", which is actually the thread-local state for things
like error reporting.

Separate the thread local state out into "threadstate".  Use the normal
subsystem initialization functions that we already have to set it up.
This makes both the global initialization system and the threadstate
system simpler to reason about.