fstab-generator: add rd.systemd.mount-extra= and friends

fstab-generator: add rd.systemd.mount-extra= and friends

Previously, mounts specified in systemd.mount-extra= are equally handled
both in initrd and the main system. So, the mounts for the main system
are also mounted in initrd.

This introduces rd.systemd.mount-extra=, which specifies mounts in initrd.
Then, mounts specified in systemd.mount-extra= are still mounted both in
initrd and the main system, but prefixed with /sysroot/ when running in
initrd.

Fixes #28516.