ivs: optionally listen for CLI commands on a unix domain socket

Reviewer: @jnealtowns

This uses the uCLI infrastructure supported by some modules. If the --cli 
option is passed without an argument, it listens on /var/run/ivs-cli.sock. You 
can easily connect to this using "socat unix-connect:/var/run/ivs-cli.sock 
readline". I'll also eventually add a standalone program to issue CLI
commands.

The bulk of the code is converting between async IO and the synchronous IO 
needed by uCLI.