Add API to register callback to set socket options

The registered callback function will be called before connect() is
called on the socket, allowing tweaking of much more than just keepalive
settings.

This deprecates xmpp_conn_set_keepalive().

Includes helper/example callback function to set default keepalive
parameters that some library users may find useful.

examples/basic.c

@@ -15,10 +15,6 @@
 
 #include <strophe.h>
 
-/* hardcoded TCP keepalive timeout and interval */
-#define KA_TIMEOUT 60
-#define KA_INTERVAL 1
-
 /* define a handler for connection events */
 static void conn_handler(xmpp_conn_t *conn,
                          xmpp_conn_event_t status,
@@ -63,7 +59,7 @@ int main(int argc, char **argv)
     xmpp_ctx_t *ctx;
     xmpp_conn_t *conn;
     xmpp_log_t *log;
-    char *jid = NULL, *password = NULL, *cert = NULL, *key = NULL, *host = NULL;
+    char *jid = NULL, *password = NULL, *host = NULL;
     long flags = 0;
     int i;
     unsigned long port = 0;
@@ -118,9 +114,6 @@ int main(int argc, char **argv)
     xmpp_conn_set_flags(conn, flags);
 
     /* setup authentication information */
-    if (cert && key) {
-        xmpp_conn_set_client_cert(conn, cert, key);
-    }
     if (jid)
         xmpp_conn_set_jid(conn, jid);
     if (password)