Allocate tlb contexts on the fly in cpu_switch, instead of statically 1 to 1

with pmaps.  When the context numbers wrap around we flush all user mappings
from the tlb.  This makes use of the array indexed by cpuid to allow a pmap
to have a different context number on a different cpu.  If the context numbers
are then divided evenly among cpus such that none are shared, we can avoid
sending tlb shootdown ipis in an smp system for non-shared pmaps.  This also
removes a limit of 8192 processes (pmaps) that could be active at any given
time due to running out of tlb contexts.

Inspired by:		the brown book
Crucial bugfix from:	tmm

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