Fix license text for several files that should be LGPLv3+

[alexey-tikhonov]

A follow up to d7999c3

Resolves: #36

[alexey-tikhonov]

@jas4711, could you please check if I missed anything?

[gemini-code-assist]

Code Review

This pull request updates the license headers across multiple files, transitioning the project from the GNU General Public License (GPL) to the GNU Lesser General Public License (LGPL). I have no feedback to provide.

[jas4711]

LGTM -- eventually the Debian QA linters will pick this up and complain further, but I think it is fine to merge meanwhile.

[alexey-tikhonov]

Note: build will be fixed by 5fc3394

[ikerexxe]

I see one additional file that can be modified, although maybe that was skipped intentionally. It's COPYING, can you confirm we don't need this change there?

[alexey-tikhonov]

I see one additional file that can be modified, although maybe that was skipped intentionally. It's COPYING, can you confirm we don't need this change there?

COPYING is a text of GPL and should be included because it is referred from COPYING.LESSER (LGPL):

  This version of the GNU Lesser General Public License incorporates
the terms and conditions of version 3 of the GNU General Public
License, supplemented by the additional permissions listed below.

[ikerexxe]

LGTM