Message 138248 - Python tracker

Patch attached:
- a new process is spawned (using assert_python_ok()) to avoid
undefined behavior (and crash on FreeBSD)
- I've kept SIGUSR1 default handler, because, as noted in
http://bugs.python.org/issue8407#msg138066 , there can be subtle
differences between default handlers and user-installed ones which can
mask bugs
- I've fixed a comment in test_sigwait