Add/update copyright notice on all C++ files

This adds a copyright notice to all C++ files and the README clarifying
the license. (There is no change of the license.) This also removes a
few names explicitly mentioned as copyright holders, instead refering to
the git log which has a full list of all osm2pgsql developers. (There is
also still the AUTHORS file which names the most important developers.)

The only exception is the sprompt.cpp file which was taken from
PostgreSQL and keeps its original license information (and, of course,
the files in contrib).

tests/test-db-copy-mgr.cpp

@@ -1,9 +1,18 @@
+/**
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ *
+ * This file is part of osm2pgsql (https://osm2pgsql.org/).
+ *
+ * Copyright (C) 2006-2020 by the osm2pgsql developer community.
+ * For a full list of authors see the git log.
+ */
+
+#include <catch.hpp>
+
 #include <string>
 #include <utility>
 #include <vector>
 
-#include <catch.hpp>
-
 #include "common-pg.hpp"
 #include "db-copy-mgr.hpp"