Add: support filtering content entries for patchpacks

This changes nothing for us, but allows patchpacks to add the
right pieces of code to start filtering content entries on patchpack
only entries.

(cherry picked from commit 77860bb61e61307e4d9cf869ba62be8ca4d9d9b3)

src/network/core/tcp_content.h

@@ -28,7 +28,11 @@ protected:
 	/**
 	 * Client requesting a list of content info:
 	 *  byte    type
-	 *  uint32  openttd version
+	 *  uint32  openttd version (or 0xFFFFFFFF if using a list)
+	 * Only if the above value is 0xFFFFFFFF:
+	 *  uint8   count
+	 *  string  branch-name ("vanilla" for upstream OpenTTD)
+	 *  string  release version (like "12.0")
 	 * @param p The packet that was just received.
 	 * @return True upon success, otherwise false.
 	 */