hwpstate(4): Ignore CurPstateLimit by default

Add a sysctl knob to allow users to re-enable it, and document the knob and
default in cpufreq.4. (While here, add a few unrelated updates to
cpufreq.4.)

It seems that the register value in some hardware simply reflects the
configured P-state. This results in an inadvertent and unintended outcome
where the P-state can only walk down, and then the driver becomes "stuck" in
the slowest possible P-state.

The Linux driver never consults this register, so that's some evidence that
ignoring the contents are relatively harmless.

PR: 234733
Reported by: sigsys AT gmail.com, Erich Dollanksy <freebsd.ed.lists AT

		sumeritec.com>