Expose outcome of select operations

[squaremo]

When you use fluxctl select, it is typically not obvious whether it succeeded or failed; and if it failed, why it failed. This is information we report in logs, so it's available; it would be useful to be able to check back on what happened.

[squaremo]

The agent now adds containers that match the service selector, but aren't addressable (because they don't map ports, for example); so, you can see some failures, at least.

Other things that might be done:

• report on the number of containers that were added (needs synchronisation -- blurgh)
• have a way of checking which containers would be matched -- i.e., a synchronous "dry-run" selection
• have a way of testing whether a specific container would be matched, or if not, why not