Unavailable-field warnings flood output

[drws]

In #57 column alignment for RTT was requested, which led to incrased flexibility with --format. In #84 a warning about unavailable field data was introduced. But these warnings can easily flood the output, as there are lots of cases where some of the data is not available. Specifying --quiet makes only the ${ip} and ${mac} fields being available, making the problem even worse. A solution would be an unavailable-field-warning suppression switch, or possibly just printing a single combined warning at the end.