dnsviz: make the tool more robust

dnsviz itself is prone to random tracebacks or even deadlocks.

When used to analyze multiple domains at once (-f), in multiple threads (-t), even one such failure will cause the entire process to fail.

Even though it's slower and less efficient, the analysis will be more resilient if dnsviz is used to verify a single domain only, one at a time (and with a timeout for the command). The parallelization should take place in our code.