Display a hint on how to open the help when entering a unknown switch

[MoritzKn]

For example: if I enter the cp command with a wrong switch, it displays how to show the help:

$ cp -y
cp: invalid option -- 'y'
Try 'cp --help' for more information.

If you don't want to hard code this, one option would be to give the programmer the ability to specifies an 'unknown option handler'.

[hongkongkiwi]

+1 for this

[jahbini]

function allowUnknownOptions has no discernable effect. The default action on unknowns is to eat the next option. calling allowUnknownOptions(false) should abort processing and show help if any funny flags are present.

[willxy]

+1 for this. I want to be able to print out usage help when an unrecognized option is given.

[shadowspawn]

This issue has not had any activity in over six months. It isn't likely to get acted on due to this report.

Feel free to open a new issue if it comes up again, with new information and renewed interest.

Thank you for your contributions.

[shadowspawn]

Related: .showHelpAfterError() added in prerelease v8.0.0-2 by PR #1534