MINOR: cli: Let configure the dynamic cookies from the cli.

This adds 3 new commands to the cli :
enable dynamic-cookie backend <backend> that enables dynamic cookies for a
specified backend
disable dynamic-cookie backend <backend> that disables dynamic cookies for a
specified backend
set dynamic-cookie-key backend <backend> that lets one change the dynamic
cookie secret key, for a specified backend.
diff --git a/include/proto/proxy.h b/include/proto/proxy.h
index a0fa454..72f1e1d 100644
--- a/include/proto/proxy.h
+++ b/include/proto/proxy.h
@@ -58,6 +58,7 @@
 int get_backend_server(const char *bk_name, const char *sv_name,
 		       struct proxy **bk, struct server **sv);
 struct proxy *cli_find_frontend(struct appctx *appctx, const char *arg);
+struct proxy *cli_find_frontend(struct appctx *appctx, const char *arg);
 
 /*
  * This function returns a string containing the type of the proxy in a format