Add support for sending remote commands to urd daemon

It would be very useful to include a nornsctl_command(COMMAND, ARGS) function to the administrative API. This way, administrative clients such as SLURM could communicate more effectively for the daemon.

Current use cases:

• Stop accepting new requests (e.g. when SLURM needs to restart a compute node).
• Ping (e.g. for SLURM to know a node is active; this would cause the current nornsctl_ping() call to be removed)