Don't simulate a pseudo address-space beyond VM_MAXUSER_ADDRESS that

Don't simulate a pseudo address-space beyond VM_MAXUSER_ADDRESS that
maps onto the upages.  We used to use this extensively, particularly
for ps and gdb.  Both of these have been "fixed".  ps gets the p_stats
via eproc along with all the other stats, and gdb uses the regs, fpregs
etc files.

Once apon a time the UPAGES were mapped here, but that changed back
in January '96.  This essentially kills my revisions 1.16 and 1.17.
The 2-page "hole" above the stack can be reclaimed now.