Make strerror(3) and strsignal(3) thread-safe, and add psignal(3) and psiginfo(3).

Change-Id: I426109db25e907980d6cb3a7a695796e45783b78
diff --git a/libc/upstream-netbsd/extern.h b/libc/upstream-netbsd/extern.h
new file mode 100644
index 0000000..942e237
--- /dev/null
+++ b/libc/upstream-netbsd/extern.h
@@ -0,0 +1,22 @@
+/*
+ * Copyright (C) 2012 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef _BIONIC_NETBSD_EXTERN_H_included
+#define _BIONIC_NETBSD_EXTERN_H_included
+
+// Placeholder.
+
+#endif