A+C: Tyler Bunnell (individual CLA)

Generated by addca.

R=gobot
CC=golang-dev
https://codereview.appspot.com/7432043
misc/dist: handle previous installation

The installer package will now detect a previous installation and warn the user
that the previous installation will be deleted. If the user continues, the
installer will delete the previous installation and install the package as
usual.

Fixes issue 4293.

R=adg
CC=golang-dev
https://codereview.appspot.com/7427044

Committer: Andrew Gerrand <adg@golang.org>