Rename SYSCTL kernel call to DIAGCTL

Change-Id: I1b17373f01808d887dcbeab493838946fbef4ef6

kernel/system.c

@@ -241,7 +241,7 @@ void system_init(void)
   /* System control. */
   map(SYS_ABORT, do_abort);		/* abort MINIX */
   map(SYS_GETINFO, do_getinfo); 	/* request system information */ 
-  map(SYS_SYSCTL, do_sysctl); 		/* misc system manipulation */ 
+  map(SYS_DIAGCTL, do_diagctl);		/* diagnostics-related functionality */
 
   /* Profiling. */
   map(SYS_SPROF, do_sprofile);         /* start/stop statistical profiling */