Cleanup whitespaces

Our coding conventions ask for TAB indents (corresponding to tabs which are 4 characters wide), and no trailing whitespace. Some code diverged from that over time.

Merging this will cause all open pull requests to not apply cleanly anymore, though. So either I wait with this till they are all merged, and then rework this PR. Or alternatively, we merge this, and then we rebase all open PRs (well, or at least all my open PRs -- the other two are more or less made obsolete by "my" PRs.