Use std::regex for RegexPattern

[yadij]

Pave the way for modern regex use. C++11 std::regex provides a portable API for pattern matching and allows future addition of flags to select different syntax languages at admin choice.

Some preliminary microtests show significant (e.g., 280%) drop in URL matching performance compared to regexec(3).

Older code still needing to be upgraded from legacy regcomp(3) API is not touched. It should be converted to use RegexPattern objects instead of directly using a std::regex.