Merge branch 'hn/refs-errno-cleanup'

Merge branch 'hn/refs-errno-cleanup'

Futz with the way 'errno' is relied on in the refs API to carry the
failure modes up the call chain.

* hn/refs-errno-cleanup:
  refs: make errno output explicit for read_raw_ref_fn
  refs/files-backend: stop setting errno from lock_ref_oid_basic
  refs: remove EINVAL errno output from specification of read_raw_ref_fn
  refs file backend: move raceproof_create_file() here