main: even more fixes for signal handling.

main: even more fixes for signal handling.

If the child doesn't die after the first SIGINT and the user presses ctrl-c
one more time, the main bup process would die instead of forwarding it on to
the child.  That's no good; we actually have to loop forwarding signals
until the child is really good and dead.

And if the child refuses to die, well, he's the one with the bug, not
main.py.  So main.py should stay alive too in the name of not losing track
of things.