update license clauses

COPYING

@@ -1,7 +1,33 @@
-This software is released under the GNU General Public License
-(reproduced below) with the additional exemption that compiling,
-linking, and/or using OpenSSL together with this software is
-allowed.
+
+special clause for libxrdp and librdp, both based on rdesktop
+these libraries link to openssl
+
+  This software is released under the GNU General Public License
+  (reproduced below) with the additional exemption that compiling,
+  linking, and/or using OpenSSL together with this software is
+  allowed.
+
+---
+
+special clause for xrdp, that main executable
+for linking with proprietary modules
+
+  Linking this library statically or dynamically with other modules
+  is making a combined work based on this library. Thus, the terms
+  and conditions of the GNU General Public License cover the whole
+  combination.
+
+  As a special exception, the copyright holders of this library
+  give you permission to link this library with independent modules
+  to produce an executable, regardless of the license terms of
+  these independent modules, and to copy and distribute the resulting
+  executable under terms of your choice, provided that you also meet,
+  for each linked independent module, the terms and conditions of the
+  license of that module. An independent module is a module which is
+  not derived from or based on this library. If you modify this
+  library, you may extend this exception to your version of the
+  library, but you are not obliged to do so. If you do not wish
+  to do so, delete this exception statement from your version.
 
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