Fix sending of StdOutput messages to the server

[ogarod]

Currently, the Fleetspeak client collects output from a daemon process's stdout and stderr streams at regular intervals and attempts to send that to the server (see client/daemonservice/execution/execution.go::flushOut()). However, that doesn't quite work because the messages do not have a destination set (an error is returned, which gets ignored).

We need to fix this as follows:

• Add an option to the service configuration that controls whether to send output from stdout, stderr, or both to the server
• Add a destination to the messages, so they actually get sent to the server. The destination will be configured in the service config.