go.tools/cmd/godoc: register redirect handler for /dl/
go.tools/cmd/godoc: register redirect handler for /dl/

LGTM=minux
R=golang-codereviews, gobot, bradfitz, minux
CC=golang-codereviews
https://codereview.appspot.com/107050044

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,chenjianlong/go.tools,chenjianlong/go.tools,karmakaze/go.tools