* Use pread/pwrite to avoid updating position in file descriptor
    * Emulate missing pread/pwrite on win32 using overlapped file IO

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.