[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: Clean ipv4/6_method when disconnecting

Reset ipv4/6_method accordingly after disconnecting as
next active connection might now have same protocols
in use.
[connman] ofono: set ipconfig method to unknown

When set_connected is called, method needs to be set
CONNMAN_IPCONFIG_METHOD_UNKNOWN so that no old configuration
is used in case it was defined by any earlier active context and
it is not defined by the current active context.
[connman] ofono: Set CONNMAN_IPCONFIG_METHOD_AUTO when no address defined

It is possible that IPv6.Settings-properties does not include
ip-address, gateway or nameservers when the context is made active.
This is due underlying ofono-driver/ril sending the assigned ipv4-addresses
first and then the assigned ipv6-addresses (within 3 secs).

This happens especially only with dual/ipv4v6-protocol and Ofono does not
currently have method to signal the updated values to it's clients.

However there is a way to workaround this by setting the ipv6_method
as CONNMAN_IPCONFIG_METHOD_AUTO and letting the connman find about
those ipv6-addresses (ip-address, gateway, nameservers) by itself.