Named returned values should only be used on public funcs and methods
when it contributes to the documentation.

Named return values should not be used if they're only saving the
programmer a few lines of code inside the body of the function,
especially if that means there's stutter in the documentation or it
was only there so the programmer could use a naked return
statement. (Naked returns should not be used except in very small
functions)

This change is a manual audit & cleanup of public func signatures.

Signatures were not changed if:

* the func was private (wouldn't be in public godoc)
* the documentation referenced it
* the named return value was an interesting name. (i.e. it wasn't
  simply stutter, repeating the name of the type)

There should be no changes in behavior. (At least: none intended)

Change-Id: I3472ef49619678fe786e5e0994bdf2d9de76d109
Reviewed-on: https://go-review.googlesource.com/20024
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>