DEV: udp: switch parser to getopt() instead of positional arguments

DEV: udp: switch parser to getopt() instead of positional arguments

In order to ease addition of new types of perturbations to udp-perturb,
let's first switch to getopt() and get rid of the positional arguments.
The random seed was already a conditional option of the rate which was
a conditional option as well. We add -r and -s for the rate and the
seed, and new options will follow.