nspawn: add helper settings_network_configured()

nspawn: add helper settings_network_configured()

The new helper returns whether the settings file had *any* networking
setting configured at all. We already have a similar helper
settings_private_network() which returns a similar result. The
difference is that the new helper will return true when the private
network was explicitly turned off, while the old one will only return
true if configured and enabled.

We'll reuse the helper a 2nd time later on, but even without it it makes
things a bit more readable.