s4-python: Override SIGINT handler in scripts only.

s4-python: Override SIGINT handler in scripts only.

Override the SIGINT handler in a few select cases only, rather than
doing so in one of the samba Python modules. I've done this where it
matters most; we can add this code to other scripts too if necessary.

This means that importing the 'samba' module from a third party
application does not have side-effects on the state of the signal
handlers.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=9068