SYNOPSIS
        #include <signals.h>

        int handle_external_signal(int signal)

DESCRIPTION
        If the driver receives a signal from the OS it forwards it to the
        mudlib master by calling this function. The signal received by the
        driver is given in <signal> and may be one of the following:
        SIGHUP, SIGINT, SIGTERM, SIGUSR1, SIGUSR2.

        If this function returns != 0, the driver will assume the signal has
        been dealt with and take NO further action.
        The exception is SIGTERM, which can't be handled. The driver will
        perform a graceful shutdown of the game after this function returns.

        If the master does not handle the signal (returns 0 or this function
        doe not exit), the driver will perform the following default actions:

        SIGHUP:  begin a graceful shutdown
        SIGINT:  send itself an unhandled SIGINT. This usually causes an
                 immediate and non-graceful shutdown.
        SIGUSR1: the driver will reload the master object
        SIGUSR2: the driver will re-open its debug log file
                 (this will happen the next time the driver writes to it)