Remove "sco" and "unixware" ports.

SCO OpenServer and SCO UnixWare are more or less dead platforms.
We have never had a buildfarm member testing the "sco" port, and
the last "unixware" member was last heard from in 2012, so it's
fair to doubt that the code even compiles anymore on either one.
Remove both ports.  We can always undo this if someone shows up
with an interest in maintaining and testing these platforms.

Discussion: <17177.1476136994@sss.pgh.pa.us>