-g should not overwrite update information passed with -u

Right now, if update information is passed together with `-g`, appimagetool will always guess the update information. This produces unexpected behavior for some of my projects, where I use linuxdeploy-plugin-appimage to create AppImages which require custom update information.

Both options are incompatible, so ideally, appimagetool should refuse to create an AppImage if both options are provided, and print a message.

linuxdeploy-plugin-appimage will be changed to not add `-g` if `$UPD_INFO` is provided.

See https://github.com/AppImage/AppImageKit/commit/5b5ca1a70cd84966d9084a9efb70115aa4b9fdb2#diff-22b5078e6286ae2bb4ef0316fd695255.