machinectl: tweak address output in "machinectl status"

machinectl: tweak address output in "machinectl status"

With this change we'll not show an "Addresses" field for machines that
we don't know any addresses for.

This changes print_addresses() to never suffix its output with a
newline, leaving that to the caller. That's a good idea since depending
on who the caller is, different rules apply: if no addresses are found,
then the list view still wants a newline, but the status view does not.

This also changes the function to return the number of found addresses,
which can be used to decide when to add a newline or not.