Introduce jetty-home contamination warning

[joakime]

Jetty version
10.0.x

Description
In light of the changes coming for Issue #5256
We should move the home-base-warning module (xml, and class too) into start.jar itself now.

Warn when attempting to modify the jetty-home directory.
New command line option to bypass this contamination.

[gregw]

I will implement this. I think it is sufficient to do this when automatically creating a start.d directory.