thread stage 6: Move thread stack management from the proc structure to

thread stage 6:  Move thread stack management from the proc structure to
the thread structure, cleanup the pmap_new_*() and pmap_dispose_*()
functions, and disable UPAGES swapping (if we eventually separate the kstack
from the UPAGES we can reenable it).  Also LIFO/4 cache thread structures
which improves fork() performance by 40% (when used in typical fork/exec/exit
or fork/subshell/exit situations).