Add x-elogind.makefs option for fstab

I opted to completely generate a unit for both mount points and swaps. For
swaps, it would be possible to use fixed template unit like elogind-mkswap@.service,
because there's no information passed except the device name. For mount points,
that's not possible because both the device name and file system type need to
be passed. Nevertheless, I expect that options will need to passed to both mkfs
and mkswap, in which case it'll be necessary to create units of both types
anyway.