Support named patterns or pattern interpolation in nmap-service-probes

[dmiller-nmap]

Currently, the third field of a match line in nmap-service-probes is a PCRE regex. Lots of services use the same formats for various things, but to save time coming up with regexes, we choose syntax that's easier to write, remember, or understand over syntax that could be more strict and allow quicker match failure (allowing Nmap to go to the next possible match sooner). A very good example of this would be a regex for the HTTP Date header field. We usually use .* for this, which works OK given that HTTP headers are short and the . does not (usually) match whitespace like the \r\n line terminator, but using an explicit regex might be better, since some services do not use the expected format.

Low priority.