Reduce MAX_NR_ZONES and remove useless zones.

I keep seeing zones on various platforms that are never used and wonder
why we compile support for them into the kernel.

IA64 on SGI for example only uses ZONE_DMA other IA64 platforms can
also use ZONE_NORMAL. So we have 50% useless zones. Large amounts
of memory go to waste with systems with a few hundred nodes and a few
hundred processors.

My x86_64 system seems to only use ZONE_DMA and ZONE_DMA32. I never see pages
in ZONE_NORMAL (probably because I have less than 4G memory).

And ZONE_HIGHMEM on a 64 bit system? We can address all memory. I keep
seeing all the highmem counters as zero on ia64 and x86_64 and even on
i386 systems.

Then it seems that ZONE_DMA32 is only used on x86_64 but we compile it into
the kernel for all platforms.

This patch makes ZONE_DMA32 and ZONE_HIGHMEM support optional. MAX_NR_ZONES
will be 2 for most non i386 platforms and even for i386 without CONFIG_HIGHMEM
set.

I tested this on IA64 and x86_64. Compiles fine on i386 with and without
CONFIG_HIGHMEM set.

The patchset consists of 8 patches that are following this message.

One could go even further than this patchset and also make ZONE_DMA optional
because some platforms do not need a separate DMA zone and can do DMA to all
of memory. This could reduce MAX_NR_ZONES to 1.