bitstring: Support large bit strings.

Replace int with either size_t or ssize_t (depending on context) in
order to support bit strings up to SSIZE_MAX bits in length. Since
some of the arguments that need to change type are pointers, we must
resort to light preprocessor trickery to avoid breaking existing code.

MFC after: 3 weeks
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D42698

(cherry picked from commit c56f45f2a9da7d989b79fd6c34b63100609ff9ae)