git__getenv: utf-8 aware env reader

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.

Using FindFirstFile and FindNextFile in win32 allows us to
use the directory information that is returned, instead of
us having to get the file attributes all over again, which
is a distinct cost savings on win32.