Auto merge of #246 - kamalmarhubi:openpty, r=alexcrichton

unix: Add openpty(3) and forkpty(3) for non-Apple platforms

The functions were added for Apple in #202. Adding them to other
platforms was pending an amendment to RFC 1291 to expand the scope of
libc to include libutil. The amendment was merged as
  https://github.com/rust-lang/rfcs/pull/1529