Lots of MacTCP networking stuff. I think all the code's written, but it

Lots of MacTCP networking stuff.  I think all the code's written, but it
doesn't actually work yet.  Also the telnet backend is still single-session,
and I haven't even touched the ssh one.  Oh, and the "Main" segment just
overflowed 32k, so we have extra linker incantations.  I'm off to the pub
now.

[originally from svn r156]