io: always returning true is a lousy API design, return the IO arg.

This changes imperative IO functions that don't return anything that
has meaning (e.g. always true) to return the IO object they were
called on instead, which allows chaining. Moreover, the API was
inconsitent before – the return type depended on the type of input.
Merge pull request #4656 from JuliaLang/sk/seek

io: always returning true is a lousy API design, return the IO arg.