Update Keepalive support to include unreliable sessions

Add in support for both reliable and unreliable sessions when calling
coap_context_set_keepalive().

include/coap2/coap_session.h:

Track the last ping mid in coap_session_t.

src/coap_io.c:
src/coap_session.c:

Support both reliable and unreliable sending of pings in coap_write().

src/net.c:

Determine whether RST packet response is associated with a CoAP Empty Ping
that was sent and handle accordingly.

configure.ac:
examples/client.c:
include/coap2/net.h:
man/Makefile.am:
man/coap-client.txt.in:
man/coap.txt.in:
man/coap_handler.txt.in:
man/coap_keepalive.txt.in: (New)
man/coap_observe.txt.in:

Add in a new man page (coap_keepalive).
Update documentation to remove references to TCP and reliable only.

include/coap2/coap_session.h

@@ -71,6 +71,7 @@ typedef struct coap_session_t {
   void *tls;                        /**< security parameters */
   uint16_t tx_mid;                  /**< the last message id that was used in this session */
   uint8_t con_active;               /**< Active CON request sent */
+  coap_tid_t last_ping_mid;         /**< the last keepalive message id that was used in this session */
   struct coap_queue_t *delayqueue;  /**< list of delayed messages waiting to be sent */
   size_t partial_write;             /**< if > 0 indicates number of bytes already written from the pdu at the head of sendqueue */
   uint8_t read_header[8];           /**< storage space for header of incoming message header */

include/coap2/net.h

@@ -347,12 +347,16 @@ coap_context_set_pki_root_cas(coap_context_t *context,
  * Set the context keepalive timer for sessions.
  * A keepalive message will be sent after if a session has been inactive,
  * i.e. no packet sent or received, for the given number of seconds.
- * For reliable protocols, a PING message will be sent. If a PONG has not
- * been received before the next PING is due to be sent, the session will
- * considered as disconnected.
+ * For unreliable protocols, a CoAP Empty message will be sent. If a 
+ * CoAP RST is not received, the CoAP Empty messages will get resent based
+ * on the Confirmable retry parameters until there is a failure timeout,
+ * at which point the session will be considered as disconnected.
+ * For reliable protocols, a CoAP PING message will be sent. If a CoAP PONG
+ * has not been received before the next PING is due to be sent, the session
+ * will be considered as disconnected.
  *
  * @param context        The coap_context_t object.
- * @param seconds                 Number of seconds for the inactivity timer, or zero
+ * @param seconds        Number of seconds for the inactivity timer, or zero
  *                       to disable CoAP-level keepalive messages.
  *
  * @return 1 if successful, else 0