+/* Sets the "probe interval" for 's' to 'probe_interval', in milliseconds. If
+ * this is zero, it disables the connection keepalive feature. Otherwise, if
+ * 's' is idle for 'probe_interval' milliseconds then 's' will send an echo
+ * request and, if no reply is received within an additional 'probe_interval'
+ * milliseconds, close the connection (then reconnect, if that feature is
+ * enabled). */