+ * Check if the given string is a valid receiver specifier.
+ *
+ * \param ra string of the form receiver_name [options...]
+ * \param lprp Filled in on success, undefined else.
+ *
+ * This function checks whether \a ra starts with the name of a receiver,
+ * optionally followed by options for that receiver. If a valid receiver name
+ * was found the remaining part of \a ra is passed to the receiver's config
+ * parser.
+ *
+ * If a NULL pointer or an empty string is passed as the first argument, the
+ * hhtp receiver with no options is assumed.
+ *
+ * \return On success the number of the receiver is returned. On errors, the
+ * function calls exit(EXIT_FAILURE).