Fix utmp and pty handling so that GTK never complains about running

Fix utmp and pty handling so that GTK never complains about running
set[ug]id. All privs-requiring pty operations are done at the very
start of the run, then privs are dropped before initialising GTK.
Utmp is handled by forking a still-privileged subprocess at this
point, and later asking it (through a pipe) to stamp utmp. The
subprocess cleans up utmp on exit, which has the additional
advantage that if the main pterm process suffers some sort of
unexpected termination (up to and including SIGKILL) the subprocess
can still mop up utmp.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2082 cda61777-01e9-0310-a592-d414129be87e