+/** @brief Create file descriptor for a subprocess to log to
+ * @param ev Event loop
+ * @param tag Tag for this log
+ * @return File descriptor
+ *
+ * Returns a file descriptor which a subprocess can log to. The normal thing
+ * to do would be to dup2() this fd onto the subprocess's stderr (and to close
+ * it in the parent).
+ *
+ * Any lines written to this fd (i.e. by the subprocess) will be logged via
+ * info(), with @p tag included.
+ */