contrib; dracut: centralise root= parsing, actually support root=s

So far, everything parsed root= manually, which meant that while
zfs-parse.sh was updated, and supposedly supported + -> ' ' conversion,
it meant nothing

Instead, centralise parsing, and allow:

root=
root=zfs
root=zfs:
root=zfs:AUTO

root=ZFS=data/set
root=zfs:data/set
root=zfs:ZFS=data/set (as a side-effect; allowed but undocumented)

rootfstype=zfs AND root=data/set <=> root=data/set
rootfstype=zfs AND root=         <=> root=zfs:AUTO

So rootfstype=zfs /also/ behaves as expected, and + decoding works

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13291