newlib: Fix header issues with C++ guards and implicit inclusion

* Added C++ sentinels if missed
* Used #pragma once, removed macro's with leading underscores
* Updated copyright checker to allow "BSD-2-Clause-FreeBSD AND
Apache-2.0" for newlib files
* Fixed minor compilation issues/implicit inclusions

components/newlib/platform_include/sys/un.h

@@ -3,8 +3,11 @@
  *
  * SPDX-License-Identifier: Apache-2.0
  */
-#ifndef _ESP_PLATFORM_SYS_UN_H_
-#define _ESP_PLATFORM_SYS_UN_H_
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 #define	AF_UNIX		1		/* local to host (pipes) */
 
@@ -13,4 +16,6 @@ struct sockaddr_un {
             char                     sun_path[108];             /*path name */
 };
 
-#endif // _ESP_PLATFORM_SYS_UN_H_
+#ifdef __cplusplus
+}
+#endif