git-compat-util.h: split out POSIX-emulating bits

The "git-compat-util.h" header is a treasure trove of various bits and
pieces used throughout the project. It basically mixes two different
things into one:

  - Providing a POSIX-like interface even on platforms that aren't
    POSIX-compliant.

  - Providing low-level functionality that is specific to Git.

This intermixing is a bit of a problem for the reftable library as we
don't want to recreate the POSIX-like interface there. But neither do we
want to pull in the Git-specific functionality, as it is otherwise quite
easy to start depending on the Git codebase again.

Split out a new header "compat/posix.h" that only contains the bits and
pieces relevant for the emulation of POSIX, which we will start using in
the next commit.

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

Author: pks-t