[connman] ofono: Clean ip-addresses and 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.

Moving c42d37d566061422e7c36408090d672c6df80c73 to approriate
place too.
[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.