Make setsid() call in firewall process optional.
Make setsid() call in firewall process optional.

We previously called setsid() to ensure that the SIGINT generated by
Ctrl+C went to the main sshuttle process instead of the firewall
process. With the previous commit, we gracefully shutdown if either
the sshuttle process or firewall process receives a SIGINT. Therefore,
the setsid() call is optional. We still try calling setsid() since the
preferred shutdown process involves having the signal go to the main
sshuttle process. However, setsid() will fail if the firewall process
is started with sudo and sudo is configured with the use_pty option.

sshuttle/sshuttle,sshuttle/sshuttle