appimagetool: make -g the default

[probonopd]

Make -g the default. Who does not like it can opt out.

[TheAssassin]

You can't expect anyone who does not know about this to be able to opt out properly...

[probonopd]

You can't expect anyone who does not know about this to be able to opt in properly either... and why should anyone want to opt out? It should do the "right thing automatically", without the user having to fiddle around imho.

[TheAssassin]

You enable a functionality that isn't known to work. Update information signalizes "yes, this AppImage supports AppImageUpdate", but this is something we cannot know. The .zsync files might not be available, or the project has been moved away from what you "guessed". If you think about the "what might go wrong" more than 3 seconds, you could come up with tons of scenarios where this is counterproductive.