detect: http_header_names sticky buffer keyword

detect: http_header_names sticky buffer keyword

A sticky buffer that allows content inspection on a contructed buffer
of HTTP header names. The buffer starts with \r\n, the names are
separated by \r\n and the end of the buffer contains an extra \r\n.

E.g. \r\nHost\r\nUser-Agent\r\n\r\n

The leading \r\n is to make sure one can match on a full name in all
cases.