Fix a controlling-terminal bug reported by Anthony Heading: Cygwin

doesn't have TIOCSCTTY, so my attempt to set the ctty of the child
process isn't doing anything, and only works by chance when you run
bash because bash does the thing that _will_ set the ctty, namely
opening the terminal file again without O_NOCTTY. So now we do that
too.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9638 cda61777-01e9-0310-a592-d414129be87e