Windows defines `timeval` with `long`, which we cannot
sanely cope with.  Instead, use a custom timeval struct.

Don't coalesce all errors into ENOENT.  At least identify EACCES.
All callers should be handling this case already, as the POSIX
`lstat` will return this.

Introduce `git__getenv` which is a UTF-8 aware `getenv` everywhere.
Make `cl_getenv` use this to keep consistent memory handling around
return values (free everywhere, as opposed to only some platforms).

Provide functionality to set the time on a filesystem entry,
using utimes or futimes on POSIX type systems or SetFileTime
on Win32.

Changed win32/path_w32.c to utilize NTFS' FindFirst..FindNext data instead of doing an lstat per file. Avoiding unnecessary directory opens and file scans reduces IO, improving overall performance. Effect is magnified due to NTFS being a kernel mode file system (as opposed to user mode).