Give a hint to the user how to change the zmqport if it fails with the default one

[mbrueckner-psi]

Whenever the default zmqport is already in use by somebody else the receiver says that the port is unavailable.
There should be hint to the help messages of (rx_)zmqport like this:
Use the commands zmqport and rx_zmqport to change the port.
For more information run
sls_detector_put -h zmqport
and
sls_detector_put -h rx_zmqport