Merge branch '7-add-support-for-sending-remote-commands-to-urd-daemon' into 'master'

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.

This new function allows removing the deprectated nornsctl_ping() function from
libnornsctl.so, since an equivalent functionality can now be supported using
the NORNSCTL_COMMAND_PING argument. Therefore, any such invocation:

    norns_error_t rv = nornsctl_ping();

can now be replicated as:

    norns_error_t rv = nornsctl_send_command(NORNSCTL_COMMAND_PING, NULL);

Closes #7

See merge request !2