cmd/godoc: add support for serving templates

cmd/godoc: add support for serving templates
doc: convert to use godoc built-in templates

tmpltohtml is gone, to avoid having a second copy of the code.
Instead, godoc -url /doc/go1.html will print the actual HTML
served for that URL.  "make" will generate files named go1.rawhtml
etc, which can be fed through tidy.

It can be hard to tell from the codereview diffs, but all the
tmpl files have been renamed to be html files and then
have "Template": true added.

R=golang-dev, adg, r, gri
CC=golang-dev
https://golang.org/cl/5782046