regexec: fix processing multibyte strings.

Matcher function incorrectly assumed that moffset that we get from
findmust is in bytes. Fix this by introducing a stepback function,
taking short path if MB_CUR_MAX is 1, and going back byte-by-byte,
checking if we have a legal character sequence otherwise.

PR: 153502
Reviewed by: pfg, kevans
Approved by: kib (mentor, implicit)
Differential revision: https://reviews.freebsd.org/D18297