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.

src/deprecated.c

@@ -231,3 +231,26 @@ int xmpp_vsnprintf(char *str, size_t count, const char *fmt, va_list arg)
 {
     return strophe_vsnprintf(str, count, fmt, arg);
 }
+
+/** Set TCP keepalive parameters
+ *  Turn on TCP keepalive and set timeout and interval. Zero timeout
+ *  disables TCP keepalives. The parameters are applied immediately for
+ *  a non disconnected object. Also, they are applied when the connection
+ *  object connects successfully.
+ *
+ *  @param conn a Strophe connection object
+ *  @param timeout TCP keepalive timeout in seconds
+ *  @param interval TCP keepalive interval in seconds
+ *
+ *  @note this function is deprecated
+ *  @see xmpp_conn_set_sockopt_callback()
+ *
+ *  @ingroup Deprecated
+ */
+void xmpp_conn_set_keepalive(xmpp_conn_t *conn, int timeout, int interval)
+{
+    conn->ka_timeout = timeout;
+    conn->ka_interval = interval;
+    conn->ka_count = 0;
+    xmpp_conn_set_sockopt_callback(conn, xmpp_sockopt_cb_keepalive);
+}