MEDIUM: checks: capture groups in expect regexes

MEDIUM: checks: capture groups in expect regexes

Parse back-references in comments of tcp-check expect rules.  If references are
made, capture groups in the match and replace references to it within the
comment when logging the error. Both text and binary regex can caputre groups
and reference them in the expect rule comment.

[Cf: I slightly updated the patch. exp_replace() function is used instead of a
custom one. And if the trash buffer is too small to contain the comment during
the substitution, the comment is ignored.]