Recent change unnecessarily breaks compatibility with previous versions of sd-zfs

[alaricljs]

The recently committed change #31 breaks all previous installs of sd-zfs by removing 'zfs:' interpretation and replacing it with 'ZFS=' instead of keeping the old style while implementing the new.

I recommend at least having a transition period where both are supported.

[dasJ]

If anyone feels up to implementing this, I'm up to merging it. However since I created this project I switched both to a different filesystem, and a different distro which does not use systemd in the initrd yet