fstab-generator: Honor mount.usr*= on kernel command line

This allows to configure boot loader entries for systems where the
root and usr filesystems are in different subvolumes (or even on
different drives).
mount-setup: skip relabelling when SELinux and SMACK not supported

This is also the only place where FTW_ACTIONRETVAL is used, so
this makes systemd compile without SELinux or SMACK support
when the standard library doesn't support this extension.