units: disallow manual starting of passive units

As passive units only are useful for ordering things within the initial
transaction there is no point in ever activating them manually, hence
refuse it.
fstab-generator: drop x-initrd.rootfs mount option

x-initrd.mount now has different meanings, if fstab-generator is called
in the initramfs.

initrd:/etc/fstab and x-initrd.mount defines mounts for the
initrd-root-fs.target

initrd:/sysroot/etc/fstab and x-initrd.mount defines mounts for the
initrd-fs.target