Replace all occurances of license.txt with COPYING, including naming the file COPYING.txt on Windows

contrib/gitian-descriptors/gitian-win32.yml

@@ -47,7 +47,7 @@ script: |
   mkdir -p $OUTDIR/src
   git archive HEAD | tar -x -C $OUTDIR/src
   cp $OUTDIR/src/doc/README_windows.txt $OUTDIR/readme.txt
-  cp $OUTDIR/src/COPYING $OUTDIR/license.txt
+  cp $OUTDIR/src/COPYING $OUTDIR/COPYING.txt
   export LD_PRELOAD=/usr/lib/faketime/libfaketime.so.1
   export FAKETIME=$REFERENCE_DATETIME
   export TZ=UTC

contrib/pyminer/pyminer.py

@@ -2,7 +2,7 @@
 #
 # Copyright (c) 2011 The Bitcoin developers
 # Distributed under the MIT/X11 software license, see the accompanying
-# file license.txt or http://www.opensource.org/licenses/mit-license.php.
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
 #
 
 import time