fsck,fstab-generator: be lenient about missing fsck.<type>

If fstab contains 1 for passno, treat this as an error, but only warn
briefly. If fstab doesn't contain this information, don't complain at
all.

Patch is complicated a bit by the fact that we might have the fstype specified
in fstab or on /proc/cmdline, in which case we can check if we have the appropriate
fsck tool, or not specified, or specified as auto, in which case we have to look
and check the type of the filesystem ourselves. It cannot be done before the
device appears, so it is too early in the generator phase, and it must be done
directly in fsck service.