fix error handling for pthread_sigmask

it must return errno, not -1, and should reject invalud values for how.

src/thread/pthread_sigmask.c

@@ -0,0 +1,10 @@
+#include <signal.h>
+#include <errno.h>
+#include <pthread.h>
+
+int pthread_sigmask(int how, const sigset_t *set, sigset_t *old)
+{
+	int ret = sigprocmask(how, set, old);
+	if (ret) return errno;
+	return 0;
+}