patch 9.1.1551: [security]: path traversal issue in zip.vim

patch 9.1.1551: [security]: path traversal issue in zip.vim

Problem:  [security]: path traversal issue in zip.vim (@ax)
Solution: drop leading ../ on write of zipfiles, don't forcefully
          overwrite existing files

A zip plugin which contains filenames with leading '../'  may cause
confusion as to where the content will be extracted.  Let's drop such
things and make sure we use a relative filename instead and don't
forcefully overwrite temporary files. Also, warn the user of such
things.

related: #17733

Signed-off-by: Christian Brabandt <cb@256bit.org>