Also, support spaces in go binaries locations, and document
GOROOT_BOOTSTRAP at the top.

Change-Id: I643d22df57aad9a2200cc256edd20e8c811bc70d
Reviewed-on: https://go-review.googlesource.com/74951
Run-TryBot: Filippo Valsorda <hi@filippo.io>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>