repart: Add --generate-fstab= and --generate-crypttab= options

repart: Add --generate-fstab= and --generate-crypttab= options

These can be used along with two new settings MountPoint= and
EncryptedVolume= to write fstab and crypttab entries to the given
paths respectively in the root directory that repart is operating on.

This is useful to cover scenarios that aren't covered by the
Discoverable Partitions Spec. For example when one wants to mount
/home as a separate btrfs subvolume. Because multiple btrfs subvolumes
can be mounted from the same partition, we allow specifying MountPoint=
multiple times to add multiple entries for the same partition.