Bring default logic to set nkmempages into the 21st century.

Bring default logic to set nkmempages into the 21st century.

The new logic is:
Up to 1G physmem use physical memory / 4,
above 1G add an extra 16MB per 1G of memory.

Clamp it down depending on available kernel virtual address space
 - up and including 512M -> 64MB	(macppc, arm, sh)
 - between 512M and 1024M -> 128MB	(hppa, i386, mips, luna88k)
 - over 1024M clamping to VM_KERNEL_SPACE_SIZE / 4

The result is much more malloc(9) space on 64bit archs with lots of memory
and large kva space.
Note: amd64 only has 4G of kva and therefor nkmempages is limited to 262144

As a side-effect NKMEMPAGES_MAX and nkmempages_max are no longer used.
Tested and OK miod@