cmd/dist: Reuse streq whenever possible.

Basically this cleanup replaces all the usage usages of strcmp() == 0,
found by the following command line:

$ grep -R strcmp cmd/dist | grep "0"

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/123330043

Committer: Ian Lance Taylor <iant@golang.org>