event/Loop: relicense to BSD-2

This library has been initially developed at CM4all and eventually
published here:
 https://github.com/CM4all/libcommon/tree/master/src/event/

On this copy of it, there were only minor modifications by people
other than me, and these are below the copyright threshold; only
non-epoll backends (which remain under the GPL) were developed by
somebody else.

src/event/Loop.cxx

@@ -1,5 +1,6 @@
-// SPDX-License-Identifier: GPL-2.0-or-later
-// Copyright The Music Player Daemon Project
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <mk@cm4all.com>
 
 #include "Loop.hxx"
 #include "DeferEvent.hxx"

src/event/Loop.hxx

@@ -1,5 +1,6 @@
-// SPDX-License-Identifier: GPL-2.0-or-later
-// Copyright The Music Player Daemon Project
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <mk@cm4all.com>
 
 #pragma once