Isn't required for token auth.

We might need this later if need to re-auth and original creds are
no longer valid/available.

Sometimes there is also a username field returned with the token, but not
always. It's nice to have but not needed (since we'll get it when we auth
our session) and trying to extract it requires lots of boilerplate from
the oauth lib. Let's keep it simple.

Provide a token with sufficient scopes or empty string to obtain new
token.

When obtaining a new token, use --token-port argument to specify the
redirect port. Specify 0 to manually enter the auth code (headless).

Re-arranged setup function so have session_config earlier for use with
get_access_token().

Added --enable-oauth / -j option.
Using --password / -p option will error and exit.

Token authenticated sessions cannot use keymaster. So reconnect using
the reusable credentials we just obtained. Can perhaps remove this
workaround once keymaster is replaced with login5.