Add API to list services installed on the system.

Fixes golang/go#20596

Change-Id: Ifa2f20ef15ccb962bd21d03788ce931dd45f2630
Reviewed-on: https://go-review.googlesource.com/45711Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>