interface-ip: DNS name server sorting support in resolv.conf.auto

Interface name servers when being written to resolv.conf.auto are sorted
based on the following parameters:
-Primary sorting key is interface dns_metric; name servers having lowest
interface dns_metric are listed first
-Secondary sorting key is interface metric; in case of equal interface
dns_metric name servers having lowest interface metric are listed first
-Finally alphabetical order of the interface names in case of equal
interface dns_metric and metric

In case the resolver queries the multiple servers in the order
listed; sorting is usefull in the following scenarios :
-Name resolving over a main and backup interface
-Assign priority to IPv6 name servers over IPv4 or vice versa

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>