Handle immediate step in local module instead of system driver

This fixes the problem where scheduler wasn't notified about performed
steps and it also makes the command available on all supported systems.

sys_netbsd.c

@@ -309,7 +309,7 @@ SYS_NetBSD_Initialise(void)
 
   lcl_RegisterSystemDrivers(read_frequency, set_frequency, 
                             accrue_offset, apply_step_offset,
-                            get_offset_correction, NULL /* immediate_step */,
+                            get_offset_correction,
                             NULL /* set_leap */);
 
 }