Debian bug 635570 did something horribly nonstandard (depending on a side

Debian bug 635570 did something horribly nonstandard (depending on a side
effect of "sed -e 'a\'" with no trailing line). But there's an actual user,
and it's not hard to implement, and it's not hard to implement, and there
isn't an obvious _other_ way to do it, so add the behavior and a test for it.

Fix some bad/missing comments while I was there, and add a couple TODOs.