remove dead ports?

I propose that we remove support for the following OS ports from our
source tree.  They are totally dead, definitely don't work, and/or
probably no one remembers what they even were.  The code just bit rots
and is in the way of future improvements.

* Dead/remove:

dgux
nextstep
sunos4
svr4
ultrix4
univel

* Dubious, but keep for now:

bsdi
irix
osf
sco

* No concern:

aix
cygwin
darwin
freebsd
hpux
linux
netbsd
openbsd
solaris
unixware
win32