Clarify effective GPLv3 licensing

While man-db's own files are individually licensed under GPLv2 (or in a
few cases LGPLv2.1), the project as a whole is effectively GPLv3 due to
Gnulib.  This was already explained in README.md, but make it a bit
clearer by having the top-level COPYING file be a copy of GPLv3.

* COPYING: Move to ...
* COPYING.GPLv2: ... here.
* COPYING: New file.
* Makefile.am (dist_noinst_DATA): Add COPYING.GPLv2.
* lib/README, man/it/man1/apropos.man1, man/it/man1/man.man1,
man/it/man1/manpath.man1, man/it/man1/whatis.man1,
man/it/man1/zsoelim.man1, man/it/man5/manpath.man5,
man/it/man8/accessdb.man8, man/it/man8/catman.man8,
man/it/man8/mandb.man8, man/man1/apropos.man1, man/man1/lexgrog.man1,
man/man1/man-recode.man1, man/man1/man.man1, man/man1/manconv.man1,
man/man1/manpath.man1, man/man1/whatis.man1, man/man1/zsoelim.man1,
man/man5/manpath.man5, man/man8/accessdb.man8, man/man8/catman.man8,
man/man8/mandb.man8, manual/comp.me, manual/db.me, manual/files.me,
manual/format.me, manual/glossary.me, manual/intro.me, manual/leader.me,
manual/man_db.me, manual/misc.me, manual/sec.me: Refer to COPYING.GPLv2
rather than COPYING.

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