Add support for PBVM addresses. In a nutshell this means:
o get the physical address and size of the PBVM page table. This

can be found in the bootinfo structure, of which the physical
address is recorded as the ELF entry point.

o translate region 4 virtual addresses to physical addresses using

the PBVM page table.

In _kvm_kvatop() make the distinction between physical address and
core file offset a little clearer to avoid confusion. To further
enhance readability, always store the translated address into pa
so that it's obvious how the translation from va to pa happened.

Approved by: re (blanket)