This change fixes a packfile heap corruption that can happen when
interacting with multiple packfiles concurrently across multiple
threads. This is exacerbated by setting a lower mwindow open file limit.

This change:

* Renames most of the internal methods in pack.c to clearly indicate
  that they expect to be called with a certain lock held, making
  reasoning about the state of locks a bit easier.
* Splits the `git_pack_file` lock in two: the one in `git_pack_file`
  only protects the `index_map`. The protection to `git_mwindow_file` is
  now in that struct.
* Explicitly checks for freshness of the `git_pack_file` in
  `git_packfile_unpack_header`: this allows the mwindow implementation
  to close files whenever there is enough cache pressure, and
  `git_packfile_unpack_header` will reopen the packfile if needed.
* After a call to `p_munmap()`, the `data` and `len` fields are poisoned
  with `NULL` to make use-after-frees more evident and crash rather than
  being open to the possibility of heap corruption.
* Adds a test case to prevent this from regressing in the future.

Fixes: #5591

This change adds support for reading multi-pack-index files from the
packfile odb backend. This also makes git_pack_file objects open their
backing failes lazily in more scenarios, since the multi-pack-index can
avoid having to open them in some cases (yay!).

This change also refreshes the documentation found in src/odb_pack.c to
match the updated code.

Part of: #5399

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

This change is the first in a series to add support for git's
multi-pack-index. This should speed up large repositories significantly.

Part of: #5399

This change moves the humongous static repository with 1025 commits into
the test file, now with a more modest 16 commits.

* Change the default of the file limit to 0 (unlimited).
* Changed the heuristic to close files to be the file that contains the
  least-recently-used window such that the window is the
  most-recently-used in the file, and the file does not have in-use
  windows.
* Parameterized the filelimit test to check for a limit of 1 and 100
  open windows.

There are some cases in which repositories accrue a large number of
packfiles. The existing mwindow limit applies only to the total size of
mmap'd files, not on their number. This leads to a situation in which
having lots of small packfiles could exhaust the allowed number of open
files, particularly on macOS, where the default ulimit is very low
(256).

This change adds a new configuration parameter
(GIT_OPT_SET_MWINDOW_FILE_LIMIT) that sets the maximum number of open
packfiles, with a default of 128. This is low enough so that even macOS
users should not hit it during normal use.

Based on PR #5386, originally written by @josharian.

Fixes: #2758

If given a NULL path, write to the object path of the repository.

Add tests for the new behavior.