[Feature] Add function to *properly* print HTTP date

strftime is badly broken and polluted by brain-damaged people that
think that locales in libc is a good idea. It isn't.

src/libutil/http.c

@@ -83,7 +83,10 @@ static const rspamd_ftok_t date_header = {
 		.begin = "Date",
 		.len = 4
 };
-
+static const rspamd_ftok_t last_modified_header = {
+		.begin = "Last-Modified",
+		.len = 13
+};
 
 #define HTTP_ERROR http_error_quark ()
 GQuark
@@ -454,6 +457,11 @@ rspamd_http_check_special_header (struct rspamd_http_connection *conn,
 	else if (rspamd_ftok_casecmp (priv->header->name, &key_header) == 0) {
 		rspamd_http_parse_key (priv->header->value, conn, priv);
 	}
+	else if (rspamd_ftok_casecmp (priv->header->name, &last_modified_header) == 0) {
+		priv->msg->last_modified = rspamd_http_parse_date (
+				priv->header->value->begin,
+				priv->header->value->len);
+	}
 }
 
 static gint
@@ -2443,3 +2451,17 @@ rspamd_http_message_parse_query (struct rspamd_http_message *msg)
 
 	return res;
 }
+
+
+glong
+rspamd_http_date_format (gchar *buf, gsize len, time_t time)
+{
+	struct tm tms;
+
+	tms = *gmtime (&time);
+
+	return rspamd_snprintf (buf, len, "%s, %02d %s %4d %02d:%02d:%02d GMT",
+			http_week[tms.tm_wday], tms.tm_mday,
+			http_month[tms.tm_mon], tms.tm_year + 1900,
+			tms.tm_hour, tms.tm_min, tms.tm_sec);
+}

src/libutil/http.h

@@ -55,14 +55,15 @@ struct rspamd_http_header {
 struct rspamd_http_message {
 	rspamd_fstring_t *url;
 	rspamd_fstring_t *host;
-	unsigned port;
 	rspamd_fstring_t *status;
 	struct rspamd_http_header *headers;
 	rspamd_fstring_t *body;
 	rspamd_ftok_t body_buf;
 	struct rspamd_cryptobox_pubkey *peer_key;
-	enum http_parser_type type;
 	time_t date;
+	time_t last_modified;
+	unsigned port;
+	enum http_parser_type type;
 	gint code;
 	enum http_method method;
 	gint flags;
@@ -357,4 +358,13 @@ void rspamd_http_router_free (struct rspamd_http_connection_router *router);
  */
 GHashTable* rspamd_http_message_parse_query (struct rspamd_http_message *msg);
 
+/**
+ * Prints HTTP date from `time` to `buf` using standard HTTP date format
+ * @param buf date buffer
+ * @param len length of buffer
+ * @param time time in unix seconds
+ * @return number of bytes written
+ */
+glong rspamd_http_date_format (gchar *buf, gsize len, time_t time);
+
 #endif /* HTTP_H_ */