The option now also returns the type, host and port of each scanner.

This adds the scanner types "GMP" and "OSP-Sensor" to these options as
well as the option to use the numeric type value used in GMP.
Error messages when using the options are now always sent to stderr and
are more specific about credential issues.

The new types and use of the numeric value are added to the --help
output and man page.

This allows setting the credential of a scanner to an existing one or to
unset the credential.

Instead of numbers the command will now return the types as strings as
accepted by --create-scanner and --modify-sensor.