sd-dhcp6-client: Fix DHCPv6 client file descriptor handling

sd-dhcp6-client: Fix DHCPv6 client file descriptor handling

The DHCPv6 client will set its state to DHCP6_STATE_STOPPED if
an error occurs or when receiving an Information Reply DHCPv6
message. Once in DHCP6_STATE_STOPPED, the DHCPv6 client needs
to be restarted by calling sd_dhcp6_client_start().

As of pull request #7796 client_reset() no longer closes the
network socket, thus a call to sd_dhcp6_client_start() needs to
check whether the file descriptor already exists in order not to
create a new one. Likewise, a call to sd_dhcp6_client_unref()
must now close the network socket as client_reset() is not
closing it.

Reported by asavah and Yu Watanabe.