tidy-up: miscellaneous

- gnutls, mbedtls: fix casing in log messages.
- src/tool_cfgable.h: drop unused header.
- appveyor.sh: variable style.
- cmakelint.sh: sync with libssh2, catch `.cmake.in` explicitly.
- examples: drop obsolete comments, exclamation marks.
- fix comment typos, casing.

Closes #19839

scripts/cmakelint.sh

@@ -52,7 +52,7 @@ cd "$(dirname "$0")"/..
     # strip off the leading ./ to make the grep regexes work properly
     find . -type f | sed 's@^\./@@'
   fi
-} | grep -E '(^CMake|/CMake|\.cmake$)' | grep -v -E '(\.h\.cmake|\.c)$' \
+} | grep -E '(^CMake|/CMake|\.cmake$|\.cmake\.in$)' | grep -v -E '(\.h\.cmake|\.c)$' \
   | xargs \
   cmake-lint \
     --suppress-decorations \