This feature should make it easier to look at very large
directory trees.

- a new mode (URL: /pkg/?m=flat) shows directory listings w/o
  indentation and entries with full path (html and text mode)
- in text mode, hierarchical (non-flat) directory listings are
  now presented with indentation (/pkg/?m=text)
- in html mode, hierarchical (non-flat) directory listings are
  presented with slightly less indentation
- there is an internal hook for programmatic control of the
  display mode (for specialized versions of godoc).

R=bradfitz
CC=golang-dev, rsc
https://golang.org/cl/5410043