pty backend now supports the changed function interface, so pterm

pty backend now supports the changed function interface, so pterm
now compiles and runs again after the major destabilisation.
Unfortunately it wasn't feasible to actually encapsulate all of the
pty backend's data, since the utmp helper and the need to fork and
drop privileges before doing anything else at all rather confuses
matters. So the data handle passed around to the pty backend is a
null pointer, and the pty backend is just as global-ridden as it
always has been. Shame, but such is life.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2128 cda61777-01e9-0310-a592-d414129be87e