Since 11ec7ce, journald isn't setting the ACLs properly anymore if
the files had no ACLs to begin with: acl_set_fd fails with EINVAL.
An ACL with ACL_USER or ACL_GROUP entries but no ACL_MASK entry is
invalid, so make sure a mask exists before trying to set the ACL.
The vacuum code used to stop vacuuming after one deletion, even
when max_use was still exceeded.
Also make usage a uint64_t, as the code already pretends it is one.
Signed-off-by: Jan Alexander Steffens (heftig) <jan.steffens@gmail.com>