Rework acpi_cpu_idle() to select the next idle state before sleeping, not
after. Unify the paths for all Cx states. Remove cpu_idle_busy and
instead do the little profiling we need before re-enabling interrupts.
Use 1 quantum as estimate for C1 sleep duration since the timer interrupt
is the main reason we wake.

While here, change the cx_history sysctl to cx_usage and report statistics
for which idle states were used in terms of percent. This seems more
intuitive than counters. Remove the cx_stats structure since it's no
longer used. Update the man page.

Change various types which do not need explicit size.