Cleanup of free_area_init code

This patch cleans up free_area_init stuff, and undefines mem_map
and max_mapnr for discontigmem, where they were horrible kludges
anyway ... we just use the lmem_maps instead, which makes much
more sense. It also kills pgdat->node_start_mapnr, which is tarred
with the same brush.

It breaks free_area_init_core into a couple of sections, pulls
the allocation of the lmem_map back into the next higher function,
and passes more things via the pgdat. But that's not very interesting,
the objective was to kill mem_map for discontigmem, which seems to
attract bugs like flypaper. This brings any misuses to obvious 
compile-time errors rather than wierd oopses, which I can't help
but feel is a good thing.

It does break other discontigmem architectures, but in a very
obvious way (they won't compile) and it's easy to fix. I think
that's a small price to pay ... ;-) At some point soon I will 
follow up with a patch to remove free_area_init_node for the 
contig mem case, or at the very least rename it to something 
more sensible, like __free_area_init.

Christoph has grander plans to kill mem_map more extensively
in addition to the attatched, but I've heard nobody disagree 
that it should die for the discontigmem case at least.

Oh, and I renamed mem_map in drivers/pcmcia/sa1100 to 
pc_mem_map because my tiny little brain (and cscope) find
it confusing like that.

Tested on 16-way NUMA-Q with discontigmem + NUMA support
and on a standard PC (well, boots and appears functional).
On top of 2.5.33-mm4

M.