reftable: introduce "reftable-system.h" header

reftable: introduce "reftable-system.h" header

We're including a couple of standard headers like <stdint.h> in a bunch
of locations, which makes it hard for a project to plug in their own
logic for making required functionality available. For us this is for
example via "compat/posix.h", which already includes all of the system
headers relevant to us.

Introduce a new "reftable-system.h" header that allows projects to
provide their own headers. This new header is supposed to contain all
the project-specific bits to provide the POSIX-like environment, and some
additional supporting code. With this change, we thus have the following
split in our system-specific code:

  - "reftable/reftable-system.h" is the project-specific header that
    provides a POSIX-like environment. Every project is expected to
    provide their own implementation.

  - "reftable/system.h" contains the project-independent definition of
    the interfaces that a project needs to implement. This file should
    not be touched by a project.

  - "reftable/system.c" contains the project-specific implementation of
    the interfaces defined in "system.h". Again, every project is
    expected to provide their own implementation.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>