Add a systemd generator for root=nbd:.. so that systemd has a correct
sysroot.mount unit.
Use export names instead of port numbers, because port number based
exports are deprecated and were removed.
The following change makes the check for the nbd port or named export
more robust.
I wasn't sure whether to include sed in the dracut_install() of
module-setup.sh since net already does that (and nbd depends on that).
Name based connects fail because of the quotes around the $nbdport.
For name based connects, the -N option also gets included. For
instance nbd-client 192.168.0.1 '-N ltsp' /dev/nbd0.
I believe the quotes are not necessary for actual port numbers.