[#964,!577] Added socket descriptor parameter to IfaceMgr external callback handler

[#964,!577] Added socket descriptor parameter to IfaceMgr external callback handler

src/lib/dhcp/iface_mgr.*
    typedef SocketCallback - added socket descriptpor parameter
    Added socket fd to invocations of socket ready callback

src/bin/dhcp4/dhcp4to6_ipc.*
    void Dhcp4to6Ipc::handler(int /* fd */)

src/bin/dhcp6/dhcp6to4_ipc.*
    void Dhcp6to4Ipc::handler(int /* fd */)

     Dhcp6to4Ipc& ipc = Dhcp6to4Ipc::instance();
     Pkt6Ptr pkt;

src/hooks/dhcp/high_availability/ha_service.*
    socketReadyHandler() - initial stub implementation of socket ready handler