Switch back to #define for PHYSTOP.  Using the E820 to retrieve the memory map is too complicated (must be done in 16-bit real-mode, probably enlarged bootblock beyond 512 bytes, and a #define requires less explanation).

Get rid of last instances of linear address and "la"
Get ready for detecting physical memory dynamically

Allocate proper kernel page table immediately in main using boot allocator
Remove pginit
Simplify address space layout a tiny bit
More to come (e.g., superpages to simplify static table)