reftable/system: add abstraction to mmap files

reftable/system: add abstraction to mmap files

In our codebase we have a couple of wrappers around mmap(3p) that allow
us to reimplement the syscall on platforms that don't have it natively,
like for example Windows. Other projects that embed the reftable library
may have a different infra though to hook up mmap wrappers, but these
are currently hard to integrate.

Provide the infrastructure to let projects easily define the mmap
interface with a custom struct and custom functions.

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