schedcpu, uvm_meter(9): make uvm_meter() an independent timeout

schedcpu, uvm_meter(9): make uvm_meter() an independent timeout

uvm_meter(9) should not base its periodic uvm_loadav() call on the UTC
clock.  It also no longer needs to periodically wake up proc0 because
proc0 doesn't do any work.  schedcpu() itself may change or go away,
but as kettenis@ notes we probably can't completely remove the concept
of a "load average" from OpenBSD, given its long Unix heritage.

So, (1) remove the uvm_meter() call from schedcpu(), (2) make
uvm_meter() an independent timeout started alongside schedcpu() during
scheduler_start(), and (3) delete the vestigial periodic proc0 wakeup.

With input from deraadt@, kettenis@, and claudio@.  deraadt@ cautions
that this change may confuse administrators who hold the load average
in high regard.

Thread: https://marc.info/?l=openbsd-tech&m=168710929409153&w=2

general agreement with this direction from kettenis@
ok claudio@