Also:
- rename a spellcheck file to match curl.
- editorconfig: fix line width.
- editorconfig: make it use UTF-8.
- editroconfig: apply some rules to all files.
- .gitignore: drop dupe, drop `.DS_Store` (not created by this repo),
sort.
- .gitignore: add for tests executables.
Closes#1718
Primarily this is handling cases where top-level files moved into
the docs/ directory. I also corrected a typo and removed the
claim that libssh2 is public domain.