MFC r335773: Revert preference to be an int.

While in base we use it as a boolean (of the wrong spelling), there's
at least one out of tree user that needs it to be int since priorirty
is a small int, not a 0/1. In deference to the time it's wasted me and
my team, push this up into FreeBSD for whatever short life boot1 may
have in the tree.