[connman] ofono: Clean nameservers when disconnecting

Nameservers need to be cleaned after
disconnecting so that they are not reused when
pdp-protocol e.g. changes from ipv6 to dual/ipv4v6.
[connman] ofono: make sure valid addresses/methods are used in set_connected

When connection is set connected, method needs to be set
CONNMAN_IPCONFIG_METHOD_UNKNOWN so that no old configuration
is used in case only ipv4/ipv6-configuration is set.

IPv6 can be also CONNMAN_IPCONFIG_METHOD_AUTO with dual/ipv4v6.