Colin Watson reports that gnome-session has been known to leave

SIGPIPE ignored in its child processes, leading to unexpected
behaviour inside pterms. (The gnome-session I'm sitting in front of
doesn't seem to do this as far as I can tell, but I don't doubt there
are some that do.) Add SIGPIPE to the list of signals we reset to
default behaviour before launching pterm's child process.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9117 cda61777-01e9-0310-a592-d414129be87e