knot-resolver: fix binding to interfaces

This PR tries to fix #196 (closed)

It introduces new behavior in case there is no interface set in /etc/config/resolver. It will try to bind to available IP addresses (with exception of IPv6 link-local address) Also, it now use net.listen function instead of -a parameter.

@vcunat Could you please share your opinion about this approach or how it could be done better?