godoc: don't convert multi-line functions into one-liners by default

- new heuristic: if both the opening { and closing } braces are on the
  same line, and the function body doesn't contain comments or is other-
  wise too long (e.g. signature too long), it is formatted as a one-line
  function

- related cleanups along the way

- gofmt -w src misc led to no additional changes as expected

R=rsc, rsc1
CC=golang-dev, ken2, r
https://golang.org/cl/758041