mount: don't consider activated until /sbin/mount returns

mount: don't consider activated until /sbin/mount returns

So far, we considered mount units activated as soon as the mount
appeared.  This avoided seeing a difference between mounts started by
systemd, and e.g. by running `mount` from a terminal.
(`umount` was not handled this way).

However in some cases, options passed to `mount` require additional
system calls after the mount is successfully created.  E.g. the
`private` mount option, or the `ro` option on bind mounts.
It seems best to wait for mount to finish doing that.  E.g. in
the `private` case, the current behaviour could theoretically cause
non-deterministic results, as child mounts inherit the
private/shared propagation setting from their parent.

This also avoids a special case in mount_reload().