Teach homed to detect OPAL and double-encrypt
Right now, systemd-homed uses subvolumes (or just plain directories) to store homedirs if the filesystem backing /home is itself encrypted.
This makes sense generally, but does not in the case of OPAL. If /home is encrypted with OPAL, there's no performance hit to double-encrpytion and we should probably default to the LUKS backend in homed
The workaround for now is to configure homed to always default to LUKS (via homed.conf). But in cases where we'd be stacking LUKS on top of LUKS, this will cause a big performance hit. We should fix this properly