kernel: avoid useless mmu flushes, implement better wait condition for procflushmmu()

procflushmmu() returns once all *OTHER* processors that had
matching processes running on them flushed ther tlb/mmu state.
the caller of procflush...() takes care of flushing "up" by
calling flushmmu() later.

if the current process matched, then that means m->flushmmu
would be set, and hzclock() would call flushmmu() again.

to avoid this, we now check up->newtlb in addition to m->flushmmu
in hzclock() before calling flushmmu().

we also maintain information on which process on what processor
to wait for locally, which helps making progress when multiple
procflushmmu()'s are running concurrently.

in addition, this makes the wait condition for procflushmmu()
more sophisticated, by validating if the processor still runs
the selected process and only if it matchatches, considers
the MACHP(nm)->flushmmu flag.

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