A slight improvement over the previous UI.

- now it's possible to select protocol and send 100% of traffic over it
  (udp, tcp, dot, doh)
- check dir exists to fail early (instead of after test)
- rename options related to binding to IP, these are likely to change in
  the future
- remove stats_interval, since it's better to always use 1s with updated
  dnsjit

Related #6