Skip all interface config questions when there are none

Skip all interface config questions when there are none

Even without any interfaces the installer still asks for one and only
continues when the only possible answer 'done' is provided.

That means one mandatory but useless answer during installations like
	# vmctl start -c -d ./install72.img -d ./disk.img test
	...
	System hostname? (short form, e.g. 'foo') test

	Available network interfaces are:  .
	Network interface to configure? (name, lladdr, '?', or 'done')
	A response is required.
	Network interface to configure? (name, lladdr, '?', or 'done') ?
	Available network interfaces are:  .
	Network interface to configure? (name, lladdr, '?', or 'done') done
	DNS domain name? (e.g. 'example.com') [my.domain]
	...

Skip it when there is no viable answer:
	# vmctl start -c -d ./install72.img -d ./disk.img test
	...
	System hostname? (short form, e.g. 'foo') test

	DNS domain name? (e.g. 'example.com') [my.domain]
	...

OK deraadt