Fixes <https://issues.guix.gnu.org/58663>.
This is a followup to b665dd4a99 and
dcb640f02b, allowing "nodiratime" to be
preserved for bind mounts.
* guix/build/syscalls.scm (MS_NODIRATIME): New variable.
(option-string->mount-flags): Recognize it.
(statfs-flags->mount-flags): Likewise.