[3997] Corrected DHCP6 server crash on exit when DDNS is enabled

[3997] Corrected DHCP6 server crash on exit when DDNS is enabled

Rather that stopping it explicitly, the server was relying D2ClientMgr
to stop itself during its own destruction.  This was falling over during
process wind-down because the IfaceMgr singletone was being destroyed before
the D2ClientMgr instance.  The server destructor now explicitly stops the
D2ClientMgr.

src/bin/dhcp6/dhcp6_messages.mes
    added DHCP6_SRV_D2STOP_ERROR log message

src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::stopD2() - new method stops the NCR sender, causing it to
    unregister its WathSocket from IfaceMgr

    Dhcpv6Srv::~Dhcpv6Srv() - added call to Dhcpv6Srv::stopD2()

src/bin/dhcp6/tests/dhcp6_process_tests.sh.in
    Enabled DNS updates in the CONFIG string.  This ensures the D2
    client will be started during system tests