setup_boot_args(), setup_client_args(), setup_args() traversing
datastructures on each invocation. Even if performance is not
important to perl script (though it definitely saves some CO2), this
nonetheless provokes some code-reading questions. Reading and
debugging such code is not convenient.

The better way is to prepare all the data in advance in an easily
readable form as well as do the validation step before any further
processing.

Use mtr_report() instead of die() like the other code does.

TODO: do_args() does even more data processing magic. Prepare that
data according the above strategy in advance in pre_setup() if possible.