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/system/do_sigreturn.c

@@ -1,20 +1,10 @@
 /* The system call that is implemented in this file:
- *     SYS_SIGCTL	# signal handling functionality 
+ *   m_type:	SYS_SIGRETURN
  *
- * The parameters and types for this system call are:
- *     SIG_REQUEST 	# request to perform			(long)
- *     SIG_PROC  	# process to signal/ pending		(int)
- *     SIG_CTXT_PTR 	# pointer to sigcontext structure	(pointer)	
- *     SIG_FLAGS    	# flags for S_SIGRETURN call		(int)	
- *     SIG_MAP		# bit map with pending signals		(long)	
- *     SIG_NUMBER	# signal number to send to process	(int)	
+ * The parameters for this system call are:
+ *     m2_i1:	SIG_PROC  	# process returning from handler
+ *     m2_p1:	SIG_CTXT_PTR 	# pointer to sigcontext structure
  *
- * Supported request types are in the parameter SIG_REQUEST:
- *     S_GETSIG		# get a pending kernel signal
- *     S_ENDSIG		# signal has been processed 
- *     S_SENDSIG	# deliver a POSIX-style signal 
- *     S_SIGRETURN	# return from a POSIX-style signal 
- *     S_KILL		# send a signal to a process 
  */
 
 #include "../system.h"