r5411: make network interface selection a bit saner

r5411: make network interface selection a bit saner

- if we have no configured network interfaces, then don't start nbtd (when I add dynamic
  interface loading this will change to a delay until a network interface comes up)

- choose the best interface by netmask for torture tests that need a
  specific IP (such as the WINS test). Added iface_best_ip() for that.

- if specific interfaces are chosen in smb.conf, then keep that ordering, and
  default to the first one listed
(This used to be commit 4d08c114079ef6d1d10a96195046fe43631aefa2)