System processes can be signaled; signals are transformed in SYS_EVENT message

that passes signal map along. This mechanisms is also used for nonuser signals
like SIGKMESS, SIGKSTOP, SIGKSIG.

Revised comments of many system call handlers. Renamed setpriority to nice.

kernel/main.c

@@ -201,6 +201,9 @@ int how;				/* reason to shut down */
           return;			/* await sys_abort() from TTY */
   }
 
+  /* Send signal to TTY so that it can switch to the primary console. */
+  send_sig(TTY, SIGKSTOP);
+
   /* Allow processes to be scheduled to clean up, unless a CPU exception 
    * occurred. This is done by setting a timer. The timer argument passes
    * the shutdown status.