Rename COPYING.GPL --> COPYING.GPLv2 and COPYING.LGPL --> COPYING.LGPLv2.1.

This is in preparation for the (L)GPL license upgrade code that will add
version 3 of these licenses.

Originally committed as revision 19114 to svn://svn.ffmpeg.org/ffmpeg/trunk

LICENSE

 Most files in FFmpeg are under the GNU Lesser General Public License version 2.1
-or later (LGPL v2.1+). Read the file COPYING.LGPL for details. Some other files
-have a MIT/X11/BSD-style license. In combination the LGPL v2.1+ applies to
+or later (LGPL v2.1+). Read the file COPYING.LGPLv2.1 for details. Some other
+files have MIT/X11/BSD-style licenses. In combination the LGPL v2.1+ applies to
 FFmpeg.
 
 Some optional parts of FFmpeg are licensed under the GNU General Public License
-version 2 or later (GPL v2+). See the file COPYING.GPL for details. None of
+version 2 or later (GPL v2+). See the file COPYING.GPLv2 for details. None of
 these parts are used by default, you have to explicitly pass --enable-gpl to
 configure to activate them. In this case, FFmpeg's license changes to GPL v2+.