fstab-generator: add mode to check /sysroot/etc/fstab and maybe do daemon-reload

fstab-generator: add mode to check /sysroot/etc/fstab and maybe do daemon-reload

The idea is that we can peek into /sysroot/etc/fstab and figure out if there's
anything interesting there. We could use a separate binary for this, but we'd
need to duplicate most of the logic that in systemd-fstab-generator. Thus I
think it's nicer to make systemd-fstab-generator work as a multi-call binary.

If called as systemd-sysroot-fstab-check, we look for units that we'd mount and
call daemon-reload and initrd-fs.target/restart, similarly to what we did
before, but in the process itself.