REUSE: add copyright headers to more files, and .gitignore updates

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

vms/libssh2_make_example.dcl

@@ -1,3 +1,6 @@
+$! Copyright (C) The libssh2 project and its contributors.
+$!
+$! SPDX-License-Identifier: BSD-3-Clause
 $!
 $!
 $ olddir = f$environment("default")