Launch vnc viewer process in a new process group

So that when we send a SIGINT to the cli, it is not also sent by the terminal
to the child process and thus terminating it.

cccli/commands/vnc.py

@@ -54,6 +54,7 @@ class VNCViewer(threading.Thread):
         proc = subprocess.Popen("%s '%s::%s'" % (self.viewer,
                                                  fwd.source_ip,
                                                  fwd.source_port),
+                                preexec_fn=os.setpgrp,
                                 shell=True, close_fds=True,
                                 stdin=onull, stdout=onull, stderr=onull)
         proc.wait()