CLEANUP: cfgparse-listen: move the HTTP rule sets to their own parser

CLEANUP: cfgparse-listen: move the HTTP rule sets to their own parser

"http-request", "http-response" and "http-after-response" were handled
by three same blocks which only differed by the rule list, the parsing
function and the validity masks. They were moved to a new function
proxy_parse_http_rules(), making the keywords discoverable via "-dKcfg".

Note that a failure to parse the action now returns a fatal error
instead of immediately aborting the config parsing, so that some of
the remaining lines are still checked.