Log Message:
revert part of the previous set of changes:  acpiacad is broken on some
machines so we can't use that to detect ac-power.  for now, revert to what
we were doing before.

XXX perhaps acpiacad should be used if it exists, and if it doesn't fall
XXX back to using "acpibat.*discharging".  then people with broken acpiacad
XXX can simply not include that driver in their kernel, but worry about
XXX that again after pkgsrc freeze.

also, check __NetBSD_Version__ to see when various envsys(4) API changes
occured, and avoid using new methods on old systems. this should fix PR#23355.