env: only long options are allowed to contain '='

[stefan-bellmann]

Enhance argument validation for unset command

Fixes #6166

Description

This change improves the argument validation logic for the unset command. It specifically addresses the handling of arguments containing the '=' character and ensures that only long options (starting with '--') are allowed to contain this character.

Changes

• Add a check to validate if an argument contains the '=' character
• Implement an additional check that ensures that short options '-' used with '=' assignment are treated as invalid arguments
• Return an error with a descriptive message for invalid arguments

Impact

This change enhances the robustness of the unset command by preventing potential misuse or confusion when users provide arguments with the '=' character in short option format.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/printenv. tests/misc/printenv is passing on 'main'. Maybe you have to rebase?

[sylvestre]

please also add a test in tests/by-util/test_env.rs to make sure we don't regress