clockmod: Properly implement Intel software controlled clock modulation

clockmod: Properly implement Intel software controlled clock modulation

Compared w/ the out dated p4tcc code in i386:

- Write the MSR on the correct CPU.
- Fix errata detection.
- Enable 6.25% granularity.
- Fix logical CPU setting by introducing clock modulation domain.  Each
  domain contains logical CPUs in the same core.  When changing duty
  cycle we change all logical CPUs' duty cycle in the same domain; mainly
  to avoid model specific reaction to logical CPU duty cycle change.

It is controlled through machdep.clockmod_domX.select.  Members of the
domain are listed by machdep.clockmod_domX.members.  And the available
duty cycles are listed by machdep.clockmod_domX.available.

It is enabled in i386 and x86_64 GENERIC.

* We may want to make powerd(8) aware of clockmod(4)