libregex: implement \` and \' (begin-of-subj, end-of-subj)

libregex: implement \` and \' (begin-of-subj, end-of-subj)

These are GNU extensions, generally equivalent to ^ and $ except that the
new syntax will not match beginning of line after the first in a multi-line
expression or the end of line before absolute last in a multi-line
expression.