make sigaltstack work (missing macros in signal.h, error conditions)

src/signal/sigaltstack.c

@@ -1,8 +1,18 @@
 #include <signal.h>
+#include <errno.h>
 #include "syscall.h"
 
 int sigaltstack(const stack_t *ss, stack_t *old)
 {
-	/* depends on kernel struct matching */
+	if (ss) {
+		if (ss->ss_size < MINSIGSTKSZ) {
+			errno = ENOMEM;
+			return -1;
+		}
+		if (ss->ss_flags & ~SS_DISABLE) {
+			errno = EINVAL;
+			return -1;
+		}
+	}
 	return syscall2(__NR_sigaltstack, (long)ss, (long)old);
 }