Depending on the bearer's configuration method, the data-port is
either a networking interface, or an tty for ppp.

Let's treat them strictily separate.

Also, rework how NM_MODEM_DATA_PORT was used in both contexts.
Instead, use the that we actually care about.

Also, when nm_device_set_ip_ifindex() fails, fail activation
right away.

Also, we early try to resolve the network interface's name to
an ifindex. If that fails, the device is already gone and we
fail early.