MINOR: proxy: introduce http only options

This commit is innoffensive but will allow to do some code refactors in
existing proxy http options. Newly created http related proxy options
will also benefit from this.
diff --git a/include/haproxy/proxy-t.h b/include/haproxy/proxy-t.h
index f1c5b03..e863299 100644
--- a/include/haproxy/proxy-t.h
+++ b/include/haproxy/proxy-t.h
@@ -266,6 +266,10 @@
 	char buf[VAR_ARRAY];                    /* copy of the beginning of the message for bufsize bytes */
 };
 
+/* http options */
+struct proxy_http {
+};
+
 struct proxy {
 	enum obj_type obj_type;                 /* object type == OBJ_TYPE_PROXY */
 	char flags;                             /* bit field PR_FL_* */
@@ -441,6 +445,7 @@
 		char *elfs_file;
 		int elfs_line;
 	} conf;					/* config information */
+	struct proxy_http http;			/* http only options */
 	struct eb_root used_server_addr;        /* list of server addresses in use */
 	void *parent;				/* parent of the proxy when applicable */
 	struct comp *comp;			/* http compression */