More cleanup of the memory management code.

Estimated hours taken: 5

More cleanup of the memory management code.
This time we clean up the signal handler setup code.

runtime/Mmakefile:
	Add new files.

runtime/mercury_memory.c:
	Rename setup_signal() to setup_signals().

runtime/mercury_memory_handlers.c:
runtime/mercury_memory_handlers.h:
	Clean up signal handling.
	Use MR_setup_signal to setup signal handlers.
	Define bus_handler and segv_handler signal handlers, the
	old signal handlers are just one or the other (or both).

runtime/mercury_prof.c:
	Use MR_setup_signal to setup signal handler.

runtime/mercury_signal.c:
runtime/mercury_signal.h:
	New files -- a standard interface for setting up signal
	handlers (a porting base, if you like).

runtime/mercury_memory.c

@@ -186,7 +186,7 @@ init_memory(void)
 
 	init_memory_arena();
 	init_zones();
-	setup_signal();
+	setup_signals();
 	if (memdebug) debug_memory();
 } /* end init_memory() */