#1531 print command index bug

[D. Michael McIntyre]

Tried to print via LilyPond, first time since the distro upgrade. Failed with message about not being able to find gtk-lp, which should be index 0. Preferences dialog was showing lpr, which should be index 1. Setting the dialog to lp, should be 3, got printing to work. I have both commands but I suspect a +1 problem here and lpr was actually used. Or a settings amnesia/sync problem.

I should determine which and fix this.

[Philip Leishman]

Indeed it was a +1 problem.
The configuration was showing lp but I was actually using lpr !!
Now I still use lpr - and that is what is shown in the config.
I believe lp lpr are basically the same now (front end for CUPS).
I believe direct printing is not used a lot - it is nicer going through the preview.
See merge request.

[Ted Felix]

Pushed as [ef141a].