bitset.9: add missing MLINKS

Add MLINKS for new bitset(9) APIs in r364796 /
f8782001804f674dc3aff53053af5ebc3c199f02 and
ae4a8e52072a87bfd49d553b9b14450c626269f8.

Reported by: trasz
Reviewed by: cem
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D27944