paste: allow multiple stdin inputs

[andrewliebenow]

Multiple stdin ("-") inputs were causing a deadlock, since stdin was locked when processing the first occurrence.

Also: fix a couple other bugs in paste, and add tests for them.

[jgarzik]

Well done splitting multiple logical changes into separate commits.

[andrewliebenow]

👍🏻

Rc<Cell<Option<Box<dyn BufRead>>>> is not great, but I don't know Rust well enough to know the usual solution in these situations.