fix code style script: enforce max line width of 120 chars

This was briefly discussed by some of the devs on skype and I put it here to open the discussion to everybody. Currently some part of our code contains very long lines which makes it unreadable side by side even on a large monitor. In my opinion it makes sense to enforce a line width limit at least for new code to get this under control.

I selected 120 because we have tabwidth 8 and most (but not all) code fits 120. Also it's a moderate limit compared to 80 and leaves enough room also for people who like to write longer lines.

I don't want to kick off a religious discussion about linewidths and editors. If the general agreement is that we need a standard for this then lets merge it, otherwise I am happy to close this PR.