use netbsd <sys/signal.h> and sigset_t

. create signals-related struct message type to store sigset_t
	  directly
	. create notify-specific message types, so the generic NOTIFY_ARG
	  doesn't exist anymore
	. various related test expansions, improvements, fixes
	. add a few error-checks to sigismember() calls
	. rename kernel call specific signals fields to SYS_*

Change-Id: I53c18999b5eaf0cfa0cb25f5330bee9e7ad2b478

include/arch/earm/include/stackframe.h

@@ -1,6 +1,8 @@
 #ifndef _ARM_STACKFRAME_H
 #define _ARM_STACKFRAME_H
 
+#include <sys/types.h>
+
 typedef u32_t reg_t;         /* machine register */
 
 struct stackframe_s {