Fix for bug#5512, don't rely on SIGIO alone.

* xsmfns.c (x_session_initialize): Move initialization of ice_fd and
doing_interact here.
(ice_connection_closed): New function.
(x_session_check_input, smc_die_CB, ice_io_error_handler)
(ice_conn_watch_CB, x_session_close): Call ice_connection_closed.
(x_session_check_input): Call IceCloseConnection if IceProcessMessages
returns I/O error.
(ice_conn_watch_CB): Call add_keyboard_wait_descriptor on ice_fd,
bug #5512.