kernel_cpp: Don't import all of the "std" namespace.

This file is included, directly or indirectly, by most of the
kernel-space C++ code, and so importing the entirety of "std"
seriously pollutes the global namespace.

So instead, just import "std::nothrow", which is the only thing
we really want in the global namespace. Tested on both GCC2
and GCC7 and seems to work just fine.

While I'm here, also update the include guards and copyright
header to match the standard format used elsewhere.

headers/private/kernel/util/kernel_cpp.h

@@ -1,10 +1,10 @@
-#ifndef KERNEL_CPP_H
-#define KERNEL_CPP_H
-/* cpp - C++ in the kernel
-**
-** Initial version by Axel Dörfler, [email protected]
-** This file may be used under the terms of the MIT License.
-*/
+/*
+ * Copyright 2003-2004, Axel Dörfler, [email protected].
+ * Copyright 2004-2018, Haiku, Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ */
+#ifndef _KERNEL_UTIL_KERNEL_CPP_H
+#define _KERNEL_UTIL_KERNEL_CPP_H
 
 #ifdef __cplusplus
 
@@ -13,8 +13,7 @@
 
 #if _KERNEL_MODE || _LOADER_MODE
 
-using namespace std;
-extern const nothrow_t std::nothrow;
+using std::nothrow;
 
 // We need new() versions we can use when also linking against libgcc.
 // std::nothrow can't be used since it's defined in both libgcc and
@@ -47,4 +46,4 @@ extern void operator delete[](void* ptr, std::size_t) _NOEXCEPT;
 
 #endif	// __cplusplus
 
-#endif	/* KERNEL_CPP_H */
+#endif	/* _KERNEL_UTIL_KERNEL_CPP_H */