Handle xdisplay null

[dreua]

Prints a warning and gracefully exits instead of segmentation fault.
This can happen e.g. using SSH or in text mode.

[dreua]

Related issue: #26

[cnlohr]

Merging, also trying to merge into the rawdraw backend in cntools to further its use. Thanks, @dreua If you get a chance check out https://github.com/cnlohr/rawdraw/ and see if you can make similar mods there. It would be very cool to have more than one contributor on that project.

[dreua]

I wasn't aware of the separate rawdraw repository. Ideally rawdraw would be a git submodule of cnping, wouldn't it? meld cnping/ rawdraw/ (directories being the obvious clones) shows that there is a common codebase but it's sometimes hard to tell which version is better so merging these back together might be rather hard and possibly not worth it. Anyway, I am aware of rawdraw and watching the repository now, so I'll have a look at issues there if they come up.

[cnlohr]

It would be, the problem was both started different, and no serious effort has been made to attempt re-merge. All other projects which use rawdraw do use it as an external module.

That might be a weekend project for me one of these days.