[MAJOR] merged the 'setbe' actions to switch the backend on a regex

Sin Yu's patch to permit to change the proxy from a regex was merged
with little changes :
  - req_cap/rsp_cap are not reassigned to the new proxy, they stay
    attached to the frontend

  - the actions have been renamed "reqsetbe" and "reqisetbe" for
    "set BackEnd".

  - the buffer is not reset after the switch, instead, the headers are
    parsed again by the backend

  - in Sin's patch, it was theorically possible to switch multiple times,
    but the switching track was lost, making it impossible to apply
    server responsesin the reverse order. Now switching is limited to
    1 action (separation between frontend and backend) but the filters
    remain.

Now it will be extremely easy to add other switching conditions, such
as host matching, URI matching, etc...

There's still a hard work to be done on the logs and stats.

include/common/regex.h

@@ -38,6 +38,7 @@
 #define ACT_DENY	3	/* deny the request */
 #define ACT_PASS	4	/* pass this header without allowing or denying the request */
 #define ACT_TARPIT	5	/* tarpit the connection matching this request */
+#define ACT_SETBE	6	/* switch the backend */
 
 struct hdr_exp {
     struct hdr_exp *next;