Interface names might not be valid UTF-8. But JSON strings must be
UTF-8. iproute2 will just return invalid JSON in the case where the
interface name is not a unicode name.

We need nmci.ip to work properly also in case where we have unexpected
interface names. For that reason, there was already a fallback path that
tired to parse the text output, and not use JSON.

If we already have a non-JSON variant that MUST work, there is no point
in keeping the JSON code. Drop it and always do our manual parsing.

This also fixes the "binary=True" argument, which is supposed to return
the interface name as bytes.