Backport PR #2526: Don't kill paramiko tunnels when receiving ^C

prevents ^C from being passed along to children, bringing them down

closes  #2520

IPython/external/ssh/tunnel.py

@@ -22,6 +22,7 b' Authors'
 from __future__ import print_function
 
 import os,sys, atexit
+import signal
 import socket
 from multiprocessing import Process
 from getpass import getpass, getuser
@@ -332,7 +333,8 b' def _paramiko_tunnel(lport, rport, server, remoteip, keyfile=None, password=None'
         print ('*** Failed to connect to %s:%d: %r' % (server, port, e))
         sys.exit(1)
 
-    # print ('Now forwarding port %d to %s:%d ...' % (lport, server, rport))
+    # Don't let SIGINT kill the tunnel subprocess
+    signal.signal(signal.SIGINT, signal.SIG_IGN)
 
     try:
         forward_tunnel(lport, remoteip, rport, client.get_transport())