[bug-libunistring] relicensing libunistring to "dual LGPLv3+ or GPLv2"

[Daiki Ueno]

Hello,

It was reported that there are potential use-cases where GPL2-only
programs indirectly link to the libunistring shared library through
intermediate libraries, such as GNUTLS or libidn2.  However, that is
currently not possible because libunistring is LGPLv3+.

Bruno Haible proposed to change the license of the library from LGPLv3+
to the "LGPLv3+ or GPLv2" dual license.  I was wondering if we can
proceed with that change.

According to the AUTHORS, agreements are needed for:
 - all files: Bruno
 - unigbrk: Ben
 - gen-uni-tables, uniname: me
where Bruno and I already agree on the change.
Ben, what do you think?

If it is okay, I can make a new minor release with the licensing change,
based on the latest release.

Regards,
-- 
Daiki Ueno