bitset: Remove BITSET_FOREACH_ADVANCE

BITSET_FOREACH_ADVANCE is a helper macro for
BITSET_FOREACH_IS(SET|CLR) and wasn't meant to be used directly by
consumers.

While here, fix some whitespace and move a comment back to where it
belongs.

Fixes: 5e04571cf3cf ("sys/bitset.h: reduce visibility of BIT_* macros")
MFC after: 1 week