Trac/1417 - add pseudoterminal support to the unix library

[Bryan O'Sullivan]

This is a very simple enhancement.

   openPseudoTerminal :: IO (Fd, Fd)
   getSlaveTerminalName :: Fd -> IO FilePath

Here's the ticket:

   http://hackage.haskell.org/trac/ghc/ticket/1417

The code ought to work on Linux, Solaris, BSD, HP/UX, and Cygwin, too. 
I've compile-tested it on Linux, using both openpty and explicit open of 
/dev/ptmx.

I propose a discussion period of one week.

Thanks,

	<b