cryptsetup: use WantsMountsFor= for key/header when nofail is set

cryptsetup: use WantsMountsFor= for key/header when nofail is set

The header and keyfile are necessary only for opening the device, not
for closing, so it is not necessary to deactivate the generated
cryptsetup unit when the header or keyfile backing store are removed.

This is especially useful in the case of softreboot, when the new
mount root is setup under /run/nextroot/ but we don't want to close
the cryptsetup devices for encrypted /var/ or so, and we simply
mount it directly on /run/nextroot/var/ before the soft-reboot.