Support for control-C. Finally.
I've changed the way things work a bit:
1. netns is replaced by mnexec, a general-purpose mininet helper.
2. For interactive commands, we now use mnexec -p, which prints out
the pid, so we can kill it when someone hits control-C!
3. We close file descriptors for subshells. This might save memory,
but who knows.
4. We detach our subshells from the tty using mnexec -s; thus
control-C should not terminate everything.
5. Given 4, mn -c is now necessary if you kill mininet.