lib: opt-in signal handlers

lib: opt-in signal handlers

Instead of enabling signal handlers by default, require the user of
the library to opt-in. This is done with the call to
SCEnableDefaultSignalHandlers, which sets a flag to add the default
signal handlers.

This seems like the least invasive way to do this at this time, but it
will require some re-thinking for 9.0, especially if migrate globals
to engine instances, signal handling will need to be re-thought.

Ticket: #6814