netconfig: Provide a way to signal when the watching has stopped.

This commit makes it possible for clients of the NetConfigWatcher to
learn when the watching has really stopped.

One of the motivations for this was to enable the shutdown function
returned by v23.Init to ensure that all spawned goroutines (such as the
net config watcher) have been stopped.

Change-Id: I138e508d7dcaa81d367111075f5e9e186a6d4cfb
6 files changed