MINOR: actions: Regroup some info about HTTP rules in the same struct

MINOR: actions: Regroup some info about HTTP rules in the same struct

Info used by HTTP rules manipulating the message itself are splitted in several
structures in the arg union. But it is possible to group all of them in a unique
struct. Now, <arg.http> is used by most of these rules, which contains:

  * <arg.http.i>   : an integer used as status code, nice/tos/mark/loglevel or
                     action id.
  * <arg.http.str> : an IST used as header name, reason string or auth realm.
  * <arg.http.fmt> : a log-format compatible expression
  * <arg.http.re>  : a regular expression used by replace rules