Encapsulated most of the pty backend's variables into a proper data

Encapsulated most of the pty backend's variables into a proper data
structure, in preparation for wanting more than one of them in a
single process. This can't be done cleanly, because the whole
business with pty_pre_init pre-allocating the pty rather assumes we
want a known number of the things before we drop privileges; so
there's a horrid hack to make pty_pre_init work on platforms that
have at most one pty instance per process, but at the same time
things ought to work sensibly with more than one per process _if_
pty_pre_init isn't required.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5261 cda61777-01e9-0310-a592-d414129be87e