Use GPLv2+ plus exception as license for release.

Use GPLv2+ plus exception as license for release.

Return back to GPLv2+, until the text of the exceptions is
finalized, reverting the change from 2007-07-03 and the first
part of the change from 2007-07-20.

Also:
* COPYING: Revert to GPLv2.
* COPYINGv3: New file, since some auxiliary build tools, used for
building autoconf and not installed, are GPLv3.
* Makefile.am (EXTRA_DIST): Distribute COPYINGv3.
* NEWS: Remove mention of GPLv3.
* README: Clarify situation regarding GPLv3.

Signed-off-by: Eric Blake <ebb9@byu.net>