Change hw.perfpolicy=auto by default, at startup. If the system has AC

Change hw.perfpolicy=auto by default, at startup.  If the system has AC
power connected (default is yes when no driver differentiates) then default
to 100% performance. On battery, use the existing auto algorithm (which is
admittedly somewhat unrefined).
This change overrides the system/BIOS speed and puts OpenBSD in control.
As this happens very early during boot, besides speedups in all usage usage
patterns, some surprises: unhibernate and sysupgrade times are cut in half.
note: on a few architectures, the setperf fn pointer is changed late, and
thus the auto algorithm stops timeing out.  kettenis and i will look for
a solution.
in snaps for more than a week.
ok kettenis