[MEDIUM] appsession: add "len", "prefix" and "mode" options

To sum up :
- len : it's now the max number of characters for the value, preventing
  garbaged results.
- a new option "prefix" is added, this allows to use dynamic cookie
  names (e.g. ASPSESSIONIDXXX).

Previously in the thread, I wanted to use the value found with
"capture cookie" but when i started to update the documentation, I
found this solution quite weird. I've made a small rework to not
depend on "capture cookie".

- There's the posssiblity to define the URL parser mode (path parameters
  or query string).
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 34fccdf..d5a7306 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -75,7 +75,7 @@
 int apply_filter_to_req_line(struct session *t, struct buffer *req, struct hdr_exp *exp);
 int apply_filters_to_request(struct session *t, struct buffer *req, struct hdr_exp *exp);
 int apply_filters_to_response(struct session *t, struct buffer *rtr, struct hdr_exp *exp);
-void manage_client_side_appsession(struct session *t, const char *buf);
+void manage_client_side_appsession(struct session *t, const char *buf, int len);
 void manage_client_side_cookies(struct session *t, struct buffer *req);
 void manage_server_side_cookies(struct session *t, struct buffer *rtr);
 void check_response_for_cacheability(struct session *t, struct buffer *rtr);