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/proc.h

@@ -87,7 +87,7 @@ struct proc {
 #define BEG_USER_ADDR (&proc[NR_TASKS])
 #define END_PROC_ADDR (&proc[NR_TASKS + NR_PROCS])
 
-#define NIL_PROC          ((struct proc *) 0)
+#define NIL_PROC          ((struct proc *) 0)		
 #define cproc_addr(n)     (&(proc + NR_TASKS)[(n)])
 #define proc_addr(n)      (pproc_addr + NR_TASKS)[(n)]
 #define proc_nr(p) 	  ((p)->p_nr)