Use O_NOCTTY (if available) when opening /dev/ptmx, just in case any

OS doesn't automatically assume it.

(It would seem faintly weird to me - surely opening the master end of
a given pty is a fairly good indication that you're _not_ a process
running inside it which wants to have it available as /dev/tty! But
you never know...)

git-svn-id: svn://svn.tartarus.org/sgt/putty@9727 cda61777-01e9-0310-a592-d414129be87e