NAME
        FreeSignal -- free a signal bit

   SYNOPSIS
        FreeSignal(signalNum)
                   D0

        void FreeSignal(BYTE);

   FUNCTION
        This function frees a previously allocated signal bit for reuse.
        This call must be performed while running in the same task in which
        the signal was allocated.

   WARNING
        Signals may not be allocated or freed from exception handling code.

   NOTE
        Starting with V37, an attempt to free signal -1 is harmless.

   INPUTS
        signalNum - the signal number to free {0..31}.