tun: Kill the tunnel script's process group

tun: Kill the tunnel script's process group

When invoked with --script-tun, openconnect starts the tunnel script
via "/bin/sh -c 'SCRIPT'", then sends SIGHUP to the shell's PID when
shutting down.  However, non-interactive shells are not guaranteed to
send SIGHUP to any running jobs¹; indeed, the observed behavior on
Linux is that only the shell process receives SIGHUP, and the tunnel
script continues running after openconnect exits.

A quick fix is to set the child's pgid == pid, then send SIGHUP to the
entire process group when we want to shut down.

¹ http://www.gnu.org/software/bash/manual/html_node/Signals.html

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>