First implementation of nornsctl_send_command()

The nornsctl_send_command() function allows administrative applications
such as SLURM to send control commands to the urd server to control its
behavior or to query specific information from it.

Accepted commands:
- NORNSCTL_COMMAND_PAUSE_ACCEPT: Stop accepting incoming tasks.
- NORNSCTL_COMMAND_RESUME_ACCEPT: Resume accepting incoming tasks after
  a successful NORNSCTL_COMMAND_PAUSE_ACCEPT has been received.
- NORNSCTL_COMMAND_PING: Check whether the server is up and running.