Change library license to LGPLv3+/GPLv2+.

* COPYING.GPLv2, COPYING.GPLv3, COPYING.LGPLv3: New.
* COPYING: Replace text by a reference to the new files.
* AUTHORS: Update copyright, license, and maintainer information.
* Makefile.am (EXTRA_DIST): Distribute the new files.

Our files asn1-parse.y, ans1-func.c, and asn-func.h are based on FSF
copyrighted code from GNUTLS. GNUTLS factored its ASN.1 code out into
the separate LIBTASN1 project and changed the license on 2002-06-14
from GPLv2 to LGPLv2.1. On 2007-07-04 we changed our KSBA license
from GPLv2 to GPLv3. Now we revert that license change, apply the
LIBTASN license change to LGPLv2.1, and finally switch to LPGLv3/GPLv2.