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/const.h

@@ -20,9 +20,6 @@
 #define structof(type, field, ptr) \
 	((type *) (((char *) (ptr)) - offsetof(type, field)))
 
-/* How many bytes for the kernel stack. Space allocated in mpx.s. */
-#define K_STACK_BYTES   1024	
-
 /* Constants used in virtual_copy(). Values must be 0 and 1, respectively. */
 #define _SRC_	0
 #define _DST_	1