Remove lots'o'hacks.
looutput is now static.
Other callers who want to use loopback to allow shortcutting
should call the special entrypoint for this, if_simloop(), which is
specifically designed for this purpose. Using looutput for this purpose
was problematic, particularly with bpf and trying to keep track
of whether one should be using the charateristics of the loopback interface
or the interface (e.g. if_ethersubr.c) that was requesting the loopback.
There was a whole class of errors due to this mis-use each of which had
hacks to cover them up.
Consists largly of hack removal :-)
e6febbec0e