patch 9.2.0435: [security]: backticks in 'path' may cause shell execution on completion

patch 9.2.0435: [security]: backticks in 'path' may cause shell execution on completion

Problem:  [security]: Backticks enclosed shell commands in the 'path'
          option value are executed during completion (q1uf3ng).
Solution: Skip path entries containing backticks, add P_SECURE to 'path'
          option, so that it cannot be set from a modeline (for symmetry with
          the 'cdpath' option)

Github Advisory:
https://github.com/vim/vim/security/advisories/GHSA-hwg5-3cxw-wvvg

Supported by AI.

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