+/* advertise prints fmt and ap to stderr after flushing stdout.
+ * If invo_name isn't NULL or empty, it precedes the output seperated by ": ".
+ * If what isn't NULL, errno as a string is appended.
+ * A non-empty what separates fmt from errno, surrounded by " " and ": ".
+ * BUG: No space separator before errno if what is "".
+ * If tail isn't NULL or empty then ", " and tail are appended
+ * before the finishing "\n".
+ * In summary: "invo_name: fmt what: errno, tail\n". */