MINOR: http: split the function http_action_set_req_line() in two parts

The first part is the replacement engine. It take a replacement action
number and a replacement string and process the action.

The second part is the function which is called by the 'http-request
action' to replace a request line part. This function makes the
string used as replacement.

This split permits to use the replacement engine in other parts of the
code than the request action. The Lua use it for his own http action.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 9e99ba1..080dc58 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -99,6 +99,8 @@
 int http_header_match2(const char *hdr, const char *end, const char *name, int len);
 int http_remove_header2(struct http_msg *msg, struct hdr_idx *idx, struct hdr_ctx *ctx);
 int http_header_add_tail2(struct http_msg *msg, struct hdr_idx *hdr_idx, const char *text, int len);
+int http_replace_req_line(int action, const char *replace, int len,
+                          struct proxy *px, struct session *s, struct http_txn *txn);
 void http_sess_log(struct session *s);
 void http_perform_server_redirect(struct session *s, struct stream_interface *si);
 void http_return_srv_error(struct session *s, struct stream_interface *si);