Log: Add handlers for all common signals

Log: Add handlers for all common signals

These should be safe. They just print the signal number, the stack
trace, then restore the original behavior.

The only problem is I can't do the same with SIGKILL nor SIGSTOP,
but I suppose if SIGKILL were the problem, the kernel would have left
something in the logs. And SIGSTOP would have left the process alive.

Intended for #49.