regex(3): Refactor fast/slow stepping bits in the matching engine

Adding features for matching is fairly straightforward, but this requires
some duplication because of this fast/slow setup. They can be fairly
trivially combined into a single walk(), so do it to make future additions
less error prone.

Reviewed by: cem (earlier version), emaste, pfg
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D11233