compat/mingw: split out POSIX-related bits

Split out POSIX-related bits from "compat/mingw.h" and "compat/msvc.h".
This is in preparation for splitting up "git-compat-utils.h" into a
header that provides POSIX-compatibility and a header that provides
common wrappers used by the Git project.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t