MINOR: http: split initialization

The goal is to export the http txn initialisation functions for
using it in the Lua code.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 3691074..f5aef3b 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -78,6 +78,8 @@
 int http_request_forward_body(struct stream *s, struct channel *req, int an_bit);
 int http_response_forward_body(struct stream *s, struct channel *res, int an_bit);
 void http_msg_analyzer(struct http_msg *msg, struct hdr_idx *idx);
+void http_txn_reset_req(struct http_txn *txn);
+void http_txn_reset_res(struct http_txn *txn);
 
 void debug_hdr(const char *dir, struct stream *s, const char *start, const char *end);
 int apply_filter_to_req_headers(struct stream *s, struct channel *req, struct hdr_exp *exp);