Reorganization of mercury_memory.c

[Tyson Dowd]

I need to modify mercury_memory.c to implement accurate GC, but it's a
bit of a mess (so much conditional compilation) so I'm going to
reorganize it into:
	mercury_memory_zones.{c,h}	
		The MemoryZone data type and functions
	mercury_memory_handlers.{c,h}	
		The signal handlers for seg faults, and redzones.
	mercury_memory.{c,h}
		The leftovers.

I also plan to re-organize the conditional compilation (presently, it's
almost impossible to change anything without a printout), and make the
tests more feature based.  For example, have 
	MR_CAN_GET_PC_AT_SIGNAL
rather than HAVE_MPROTECT && (HAVE_SIGINFO || PC_ACCESS).  This is
because I'm going to have to add some implementation-dependent methods
of finding these things.
	
Let me know if this is going to destory any uncommitted work (apart from
Tom, who I've already spoken with).

-- 
       Tyson Dowd           # So I asked Sarah: what's the serial number on
                            # your computer? She replied:
     trd at cs.mu.oz.au        #          A-C-2-4-0-V-/-5-0-H-Z
http://www.cs.mu.oz.au/~trd #