MEDIUM: http: implement http-request set-{method,path,query,uri}

This commit implements the following new actions :

- "set-method" rewrites the request method with the result of the
  evaluation of format string <fmt>. There should be very few valid reasons
  for having to do so as this is more likely to break something than to fix
  it.

- "set-path" rewrites the request path with the result of the evaluation of
  format string <fmt>. The query string, if any, is left intact. If a
  scheme and authority is found before the path, they are left intact as
  well. If the request doesn't have a path ("*"), this one is replaced with
  the format. This can be used to prepend a directory component in front of
  a path for example. See also "set-query" and "set-uri".

  Example :
      # prepend the host name before the path
      http-request set-path /%[hdr(host)]%[path]

- "set-query" rewrites the request's query string which appears after the
  first question mark ("?") with the result of the evaluation of format
  string <fmt>. The part prior to the question mark is left intact. If the
  request doesn't contain a question mark and the new value is not empty,
  then one is added at the end of the URI, followed by the new value. If
  a question mark was present, it will never be removed even if the value
  is empty. This can be used to add or remove parameters from the query
  string. See also "set-query" and "set-uri".

  Example :
      # replace "%3D" with "=" in the query string
      http-request set-query %[query,regsub(%3D,=,g)]

- "set-uri" rewrites the request URI with the result of the evaluation of
  format string <fmt>. The scheme, authority, path and query string are all
  replaced at once. This can be used to rewrite hosts in front of proxies,
  or to perform complex modifications to the URI such as moving parts
  between the path and the query string. See also "set-path" and
  "set-query".

All of them are handled by the same parser and the same exec function,
which is why they're merged all together. For once, instead of adding
even more entries to the huge switch/case, we used the new facility to
register action keywords. A number of the existing ones should probably
move there as well.
diff --git a/include/types/proto_http.h b/include/types/proto_http.h
index 2654b78..d9cda71 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -438,6 +438,9 @@
 			struct list key;       /* pattern to retrieve MAP or ACL key */
 			struct list value;     /* pattern to retrieve MAP value */
 		} map;
+		struct {
+			void *p[4];
+		} act;                         /* generic pointers to be used by custom actions */
 	} arg;                                 /* arguments used by some actions */
 
 	union {