MINOR: http-htx: Use a dedicated function to parse http reply arguments

MINOR: http-htx: Use a dedicated function to parse http reply arguments

A dedicated function to parse arguments and create an http_reply object is
added. It is used to parse http return rule. Thus, following arguments are
parsed by this function :

  ... [status <code>] [content-type <type>]
      [ { default-errorfiles | errorfile <file> | errorfiles <name> |
          file <file> | lf-file <file> | string <str> | lf-string <fmt> } ]
      [ hdr <name> <fmt> ]*

Because the status code argument is optional, a default status code must be
defined when this function is called.