Add keyscript mode

In keyscript mode no unlock is performed and we just print the passphrase on stdout.

This allows usage e.g. with Debian's cryptsetup-initramfs but can also be helpful when integrating with e.g. smartcards later on.

I did not make -n and -d superfluous since those can be used to print information what's being unlocked later on (e.g. if the initramfs unlocks multiple volumes.