fix protection

G/C VM_MAX_KERNEL_BUF

branches: 1.10.2; 1.10.4;
Use default PAGER_MAP_DEFAULT_SIZE for ARM and PowerPC, exception
acorn26. This bumps the KVA reservation from 4MB to 16MB and avoids
long hangs on my Cubietruck under IO. acorn26 is kept as it does have a
ridiculous low 32MB KVA limit.

branches: 1.9.12; 1.9.30;
PowerPC now exports a common view of cpu.h, vmparam.h and pmap.h
when building a MODULAR kernel or compiling _MODULE.
It should be noted that MODULAR or _MODULE export a view of the kernel
as being MULTIPROCESSOR (even if isn't).
The shared pmap TLB uses mdpg in places where it used mdpg to avoid
deadly embrance inclusion problems.

branches: 1.8.6;
Merge from uebayasi-xip:

Teach TLB miss handler (pmap_tlbmiss()) to map "Expansion ROM" area as
PA == VA. Now we don't need to reserve a TLB entry for it.

Remove incomplete, never worked dynamic run-time memory registration
(uvm_page_physload(9)). This functionality will be re-added later.

branches: 1.6.2; 1.6.4;
Remove SHMMAXPGS from all kernel configs. Dynamically compute the
initial limit as 1/4 of the physical memory. Ensure the limit is at
least 1024 pages, the old default on most platforms.

branches: 1.5.20; 1.5.28; 1.5.34;
defparam PAGER_MAP_SIZE.

branches: 1.4.18; 1.4.60; 1.4.78; 1.4.80; 1.4.84;
Use PAGE_SIZE rather than NBPG.

Make PAGE_SIZE, PAGE_SHIFT, and PAGE_MASK compile-time constants for
PowerPC processors.

Re-arrange things in evbppc & powerpc to support OEA-based eval boards
in evbppc. OEA-based board(s) to be added later.

Move ibm4xx vmparam.h to powerpc/include/ibm4xx directory. Add a stub
vmparam.h to choose the right vmparam. This is needed so evbppc can host
both ibm4xx and oea based eval boards.