patch 9.2.0300: The vimball plugin needs some love

Problem:  The vimball plugin needs some love
          (syndicate)
Solution: Clean-up, refactor and update the plugin,
          in particular, catch path traversal attacks

This change does the following
- Clean up Indentation and remove calls to Decho
- Increase minimum Vim version to 7.4 for mkdir()
- Use mkdir() consistently
- Update Metadata Header
- Remove check for fnameescape()
- Catch path traversal attacks
- Add vimball basic tests
- Remove mentioning of g:vimball_mkdir in documentation

closes: #19921

Signed-off-by: Christian Brabandt <[email protected]>

Author: chrisbra