Rename clock_systime[r|spec] to clock_systime_[ticks|timespec]

follow up the new naming convention:
https://cwiki.apache.org/confluence/display/NUTTX/Naming+of+OS+Internal+Functions

net/tcp/tcp_timer.c

@@ -477,7 +477,7 @@ void tcp_timer(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
 #endif
                           /* Update for the next probe */
 
-                          conn->keeptime = clock_systimer();
+                          conn->keeptime = clock_systime_ticks();
                           conn->keepretries++;
                         }