[#691,!395] Added bad socket purge to IfaceMgr

[#691,!395] Added bad socket purge to IfaceMgr

src/lib/dhcp/iface_mgr.cc
    IfaceMgr::purgeBadSockets() - new function to validate
    external sockets and unregister any that are invalid.

    IfaceMgr::receive4Indirect()
    IfaceMgr::receive4Direct()
    IfaceMgr::receive6Indirect()
    IfaceMgr::receive6Direct() - added logic to all
    purgeBadSockets() when select fails with EBADF

src/lib/dhcp/tests/iface_mgr_unittest.cc
    TEST_F(IfaceMgrTest, purgeExternalSockets4)
    TEST_F(IfaceMgrTest, purgeExternalSockets6) -  new tests
    to verify bad socket purging

src/lib/http/client.cc
    Move close_callback_ reset back to Connection::close()