Merge branch 'ps/reftable-portability'

Update reftable library part with what is used in libgit2 to improve
portability to different target codebases and platforms.

* ps/reftable-portability:
  reftable/system: add abstraction to mmap files
  reftable/system: add abstraction to retrieve time in milliseconds
  reftable/fsck: use REFTABLE_UNUSED instead of UNUSED
  reftable/stack: provide fsync(3p) via system header
  reftable: introduce "reftable-system.h" header