[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: 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 CONNMAN_IPCONFIG_METHOD_AUTO when no address defined

IPv6 configuration method is either CONNMAN_IPCONFIG_METHOD_FIXED
(ipv6-pdp) or CONNMAN_IPCONFIG_METHOD_AUTO (SLAAC/DHCPv6,
dual/ipv4v6-pdp).
[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.