libmount: accept X-mount.{owner,group,mode}=

libmount: accept X-mount.{owner,group,mode}=

Which take a user, group, and mode, respectively, and set them on the
target after mounting

This is vaguely similar to tmpfs(5)'s [ug]id= and mode= options,
but we POSIX-parse the user- and group names

Oft requested in systemd/zram-generator, since a common use-case
is to use it to create /tmp or an equivalent directory that needs
to be a=rwx,o+t (or a user's private temp that needs to be owned
by them) ‒ this is impossible without terrible hacks, cf.
https://github.com/systemd/zram-generator/issues/150,
https://github.com/systemd/zram-generator/issues/146, &c.

This started off as a Set{User,Group,Mode}= systemd mount unit,
but was poetterung into libmount options:
https://github.com/systemd/systemd/pull/22889

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Karel Zak <kzak@redhat.com>