Add a "daemon" build tag and toggle it with the already-existing "DOCKER_CLIENTONLY" build variable

This works mostly by refactoring our "main" package to be careful about what it imports based on the daemon build tag. :)

Also, I've updated Travis to test "client-only" compilation after it tests the daemon version.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)