MEDIUM: regex: add support for passing regex flags to regex_exec_match()

MEDIUM: regex: add support for passing regex flags to regex_exec_match()

This function (and its sister regex_exec_match2()) abstract the regex
execution but make it impossible to pass flags to the regex engine.
Currently we don't use them but we'll need to support REG_NOTBOL soon
(to indicate that we're not at the beginning of a line). So let's add
support for this flag and update the API accordingly.