libmount: cleanup UID and GIR parsing, add tests

libmount: cleanup UID and GIR parsing, add tests

* parse_uid_numeric() and parse_gid_numeric() assumes zero terminated
  strings, so do not require size. The function ul_strtou64() already
  checks that the number is terminated by \0

* use rc from ul_strtou64()

* always set errno if necessary

* return -errno or -EINVAL rather than generic -1

* add --parse-{uid,gid,mode} to test_mount_utils

Signed-off-by: Karel Zak <kzak@redhat.com>