Beef up editline wrapper.

We now have basic support to handle command completion as well as the ability
to add lines to the history. The console client will use this functionality.

Relates #29.
Clean up signal and keyboard processing.

The system monitor no longer takes on responsibility for both signal handling
and keystroke handling. We renamed it to signal_monitor and dropped keystroke
support. We don't need keyboard interaction on the server side and client's now
can use the editline wrapper.

Moreover, we renamed util::console to util::terminal to avoid confusion with
vast::console, the new console client.
Wire new console client. (#29)