dashboard/buildlet: make exec handler return process state in HTTP trailer

Requires Go tip to function, but compiles with old versions of Go, but
without returning the trailer, which we'll be able to detect in the
coordinator to make sure nobody used an old Go to cross-compile a
buildlet.

Change-Id: Ie5f71c0a4477563dea5f1526af10a49aa41536d6
Reviewed-on: https://go-review.googlesource.com/2159
Reviewed-by: Andrew Gerrand <adg@golang.org>
dashboard/env/openbsd-amd64: typo and wording change

Review comments from Joel in https://golang.org/cl/2156

Change-Id: Ic3679d5c103b85f249f0ddcaceb29e50313f3a4f
Reviewed-on: https://go-review.googlesource.com/2190
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Joel Sing <jsing@google.com>