Provide a mechanism for callers to expand the full path of a file in the
global configuration directory (that is to say, the home directory) even
if the file doesn't necessarily exist. This lets callers use their own
logic for building paths separate from handling file existence.
Don't try to determine when sysdirs are uninitialized. Instead, simply
initialize them all at `git_libgit2_init` time and never try to
reinitialize, except when consumers explicitly call `git_sysdir_set`.
Looking at the buffer length is especially problematic, since there may
no appropriate path for that value. (For example, the Windows-specific
programdata directory has no value on non-Windows machines.)
Previously we would continually trying to re-lookup these values,
which could get racy if two different threads are each calling
`git_sysdir_get` and trying to lookup / clear the value simultaneously.