Make again BSD ar(1) the default system ar(1), now properly handling
source upgrades by falling back to GNU ar(1) as necessary. Option
WITH_BSDAR is gone. Option _WITH_GNUAR to aid in upgrades is *not*
supposed to be set by the user.

Stop bootstrapping BSD ar(1) on the next FreeBSD_version bump, as
there are no known bugs in it. Bump FreeBSD_version to anticipate
this and to flag the switch to BSD ar(1), should it be needed for
something.

Input from: obrien, des, kaiw