Block SIGPIPE in Unix plink. In a port-forwarding run we may have lots

of local sockets and pipes all open at once, and if one of them is
uncleanly closed from the remote end we don't want the whole
application to die - we want to close that socket's SSH channel and
continue with the rest of the run.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9359 cda61777-01e9-0310-a592-d414129be87e