dashboard/buildlet: make exec handler return process state in HTTP trailer
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 <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/build,proppy/build,evandbrown/build,evandbrown/build,golang/build,golang/build,proppy/build,golang/build,proppy/build,evandbrown/build,proppy/build