machdep.kbdreset enables a shutdown by Ctrl-Alt-Del on amd64 and
i386. Stop abusing it on other archs for controling a shutdown by
pressing the soft power button:
* Add a MI sysctl hw.allowpowerdown; if set to 1 (the default) it
allows a power button shutdown.
* Make acpi(4)/acpibtn(4) honor hw.allowpowerdown.
* Switch the various power button intercepts on landisk, sgi, sparc64
and zaurus over to hw.allowpowerdown.
* Garbage collect the machdep.kbdreset sysctl on all archs other than
amd64 and i386.
ok miod@