Proper validation layer

[mattias-p]

Zonemaster CLI accepts user inputs through command line arguments.
Today the validation logic for these values is spread out.
It's non-trivial to review and verify that all these inputs are in fact
validated before they're used and that the implemented validation rules match
the documented ones.

Zonemaster CLI should have a clearly delimited validation layer where all
command line arguments must pass through and where all validation logic is
invoked.

Having this validation layer located close to the argument parsing would also
make it easier to maintain cohesive command line argument validation error
messages.