Detach shells from tty; monitoring changes.

Call netns using setsid to detach from the tty; this is
intended to fix the problem of control-C killing your network.

sendInt() doesn't actually work - hopefully we can fix this in the
future, but it's tricky since we want to detach the many shells from
our tty, but then optionally send tty interrupts to them! Ideally we
would either know the pid of the subprocess or be able to send a signal
to bash to get it to interrupt its subprocess.

waitOutput() now uses monitor. We also handle sentinels
coming in the middle of output (but default at end should
still work well.)