Kernel: make SIGKMESS target process list dynamic

The set of processes to which a SIGKMESS signal is sent whenever new
diagnostics messages are added to the kernel's message buffer, is now
no longer hardcoded. Instead, processes can (un)register themselves
to receive such notifications, by means of sys_diagctl().

Change-Id: I9d6ac006a5d9bbfad2757587a068fc1ec3cc083e

kernel/system/do_privctl.c

@@ -116,6 +116,7 @@ int do_privctl(struct proc * caller, message * m_ptr)
 	reset_kernel_timer(&priv(rp)->s_alarm_timer);	/* - alarm */
 	priv(rp)->s_asyntab= -1;			/* - asynsends */
 	priv(rp)->s_asynsize= 0;
+	priv(rp)->s_diag_sig = FALSE;		/* no request for diag sigs */
 
 	/* Set defaults for privilege bitmaps. */
 	priv(rp)->s_flags= DSRV_F;           /* privilege flags */