maint: follow recommended practice for using LGPL

maint: follow recommended practice for using LGPL

https://www.gnu.org/licenses/gpl-howto.html states:

You should also include a copy of the license itself somewhere in the
distribution of your program. All programs, whether they are released
under the GPL or LGPL, should include the text version of the GPL. In
GNU programs the license is usually in a file called COPYING.

If you are releasing your program under the LGPL, you should also
include the text version of the LGPL, usually in a file called
COPYING.LESSER. Please note that, since the LGPL is a set of
additional permissions on top of the GPL, it's important to include
both licenses so users have all the materials they need to understand
their rights.

* configure.ac (COPYING): No more games with non-git file.
* COPYING: New file, copied from gnulib.
* COPYING.LIB: Rename...
* COPYING.LESSER: ...to this.
* .gitignore: Track licenses in git.
* cfg.mk (exclude_file_name_regexp--sc_copyright_address): Tweak
rule.
* libvirt.spec.in (daemon, client, python): Reflect rename.

Signed-off-by: Eric Blake <eblake@redhat.com>