sd-dhcp6-client: Properly handle DHCPv6 client restart after resume

Whenever a Router Advertisement is received, dhcp6_configure() will be
called. A Router Advertisment can also instruct DHCPv6 to start acquiring
IPv6 addresses in manged mode, if it previously was handling only other
information. As an Router Advertisment is also received after the DHCPv6
client has resumed from a suspend, fix the function not to assume DHCPv6
is currently running, but instead try to restart it. Handle
sd_dhcp6_start() returning -EALREADY indicating that the DHCPv6 client was
already running.

Collect all client unrefs in one place to unclutter the error handling.

Fixes https://github.com/systemd/systemd/issues/963