kern: KASSERT(A && B) -> KASSERT(A); KASSERT(B)

Unlock correct mutex in an error path.
PR kern/48592 from Kengo NAKAHARA.

Revert the revision 1.5 in cpufreq(9). Instead, document that the KPI can
not be used before interrupts have been enabled. Suggested by macallan@.

back to using timespec, but convert to microseconds to prevent overflow.

Use microsecond rather than nanosecond resolution when sampling latencies.

Do not use xcall(9) in cpufreq_register(9), as it requires kthreads to be
running and may impose hacks like config_interrupts(9) in the backend.

Remove redundant assignment.

Fix bugs noted by alnsn@. Namely, preallocate the global structure in
order to avoid locking issues during deregistration.

Initialize cpufreq(9) normally from main().

Add a simple cpufreq(9).

This is slightly improved version presented on tech-kern@ with proper
locking and few additional functions, mainly required for ioctl(9)/cpuctl(8).