A function to provide the initialization count of the library; this is
subject to race conditions but is useful for a naive determination as to
whether the library has been initialized or not.

`git_buf_sanitize` is called with user-input, and wants to sanity-check
that input.  Allow it to return a value if the input was malformed in a
way that we cannot cope.

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".

Now that we've identified that our global settings really aren't global
at all, and refactored the library to match that, change global.c to
libgit2.c, which is especially nice since the prefix of the functions
matches the filename.