Several optimizations to sched_idletd():

 - Do not try to steal load from other CPUs if there was no contest switches
on this CPU (i.e. it was idle all the time and woke up just for bus mastering
or TLB shutdown). If current CPU was idle, then it is quite unlikely that some
other CPU has load to steal.  Under high I/O rate, when TLB shutdowns cause
numerous CPU wakeups, on 24-CPU system load stealing code may consume up to
25% of all CPU time without giving any benefits.
 - Change code that implements spinning for load to restart spin in case of
context switch.  Previous code periodically called cpu_idle() even under
high interrupt/context switch rate.
 - Rise spinning threshold to 10KHz, where it gives at least some effect
that may worth consumed power.

Reviewed by:	jeff@

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