change sigset_t functions to restrict to _NSIG

the idea here is to avoid advertising signals that don't exist and to
make these functions safe to call (e.g. from within other parts of the
implementation) on fake sigset_t objects which do not have the HURD
padding.

src/signal/sigaddset.c

@@ -4,7 +4,7 @@
 int sigaddset(sigset_t *set, int sig)
 {
 	unsigned s = sig-1;
-	if (s >= 8*sizeof(sigset_t) || sig-32U<3) {
+	if (s >= _NSIG-1 || sig-32U < 3) {
 		errno = EINVAL;
 		return -1;
 	}

src/signal/sigdelset.c

@@ -4,7 +4,7 @@
 int sigdelset(sigset_t *set, int sig)
 {
 	unsigned s = sig-1;
-	if (s >= 8*sizeof(sigset_t) || sig-32U<3) {
+	if (s >= _NSIG-1 || sig-32U < 3) {
 		errno = EINVAL;
 		return -1;
 	}

src/signal/sigisemptyset.c

@@ -4,6 +4,6 @@
 
 int sigisemptyset(const sigset_t *set)
 {
-	static const sigset_t zeroset;
-	return !memcmp(set, &zeroset, 8);
+	static const unsigned long zeroset[_NSIG/8/sizeof(long)];
+	return !memcmp(set, &zeroset, _NSIG/8);
 }

src/signal/sigismember.c

@@ -4,6 +4,6 @@
 int sigismember(const sigset_t *set, int sig)
 {
 	unsigned s = sig-1;
-	if (s >= 8*sizeof(sigset_t)) return 0;
+	if (s >= _NSIG-1) return 0;
 	return !!(set->__bits[s/8/sizeof *set->__bits] & 1UL<<(s&8*sizeof *set->__bits-1));
 }