lib: enable strerror and strncpy checksrc warnings in subdirs

Closes #14834

libcurl.def: move from / into lib

It is meant for the library, it belongs in lib/

Closes #14796

CHANGES: rename to CHANGES.md, no longer generated

- scripts/log2changes.pl was not included in release tarballs, which broke
reproducible builds

- since log2changes uses git

CHANGES: rename to CHANGES.md, no longer generated

- scripts/log2changes.pl was not included in release tarballs, which broke
reproducible builds

- since log2changes uses git to generate the contents, it makes it difficult
to generate the same contents later (it would need to be fixed)

- the CHANGES file has outlived its purpose. the main changes are in the
RELEASE-NOTES, the rest are better tracked directly using git or on GitHub

- put a fixed CHANGES.md in there instead pointing out where the info lives
now

Closes #14331

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reuse: fix typo in comment

Follow-up to 9104bad82004d908e1fe66a425f8ca78f975045d #14107

reuse: switch to REUSE 3.2 and REUSE.toml

- remove scripts/copyright.pl

Closes #14107