bitset: Reimplement BIT_FOREACH_IS(SET|CLR)

Eliminate the nested loops and re-implement following a suggestion from
rlibby.

Add some simple regression tests.

Reviewed by: rlibby, kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 51425cb2107c07ff379639edfbad65c77b55c3b8)