Skip options that begin with the `Safe` prefix from appearing in the
serialization of a Measurement that will be submitted to the OONI
backend.
Fixes https://github.com/ooni/probe/issues/2214
This diff refactors the ./internal/cmd/miniooni pkg and moves the code
for running experiments inside of the ./internal/oonirun pkg.
It's the first concrete step towards https://github.com/ooni/probe/issues/2184.
