This module implements set_sig_handler(), which sets up a signal handler and
(optionally) returns an object which causes the signal handler to be reset to
the previous value, when it goes out of scope.

Also implemented is timeout_call() which takes a timeout value and a code
reference, and executes the code reference wrapped with an alarm timeout.

Finally, two convenience routines are defined which allow one to get the signal
name from the number -- sig_name(), and get the signal number from the name --
sig_number().