[connman] ofono: Implement protocol-message handling for context_changed

Reset ipv4/6_method accordingly when protocol is set
ip/ipv6/dual on ofono, which can be only done when context
is not active.
[connman] ofono: SLAAC/DHCPv6 is the only supported method (RFC 6459)

Only valid and supported method setting IPv6 configuration
is CONNMAN_IPCONFIG_METHOD_AUTO (SLAAC/DHCPv6).
[connman] fix service structure access while unreferencing

It's unsafe to access service struct after possibly unreferencing it
for the last time, so needed to change the order of statements in
stop_recurring_online_check() so that access after last unref isn't
possible.
[connman] network: protect service structure access with reference

As __connman_connection_gateway_remove() may unref service, wrap
network.c:set_disconnected() with connman_service_ref/unref to make
sure the struct stays around during the function call.
[connman] ofono: make sure valid addresses/methods are used in set_connected

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

Only supported method for IPv6 is CONNMAN_IPCONFIG_METHOD_AUTO (RFC 6459).
[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.