cmd/go: always show current value for GO15VENDOREXPERIMENT

cmd/go: always show current value for GO15VENDOREXPERIMENT

Prior behavior would show empty string when unset. In go1.5 this
would result in "off". In go1.6 this will result in "on". This
change will make empty or "0" off and "1" on for go1.5 and go1.6.
Vendor tools can then rely on this value.

Discussion:
https://groups.google.com/forum/#!topic/golang-dev/oZzcXrlRrkA

Change-Id: I7e145a32e813dfde02dc262a9186c7af28db7b92
Reviewed-on: https://go-review.googlesource.com/17487
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>