declaration.
(server_x11_callback): Renamed (was server_x11_forward), and
enabled.
(server_x11_socket): New attribute display_number.
(open_x11_socket): Pass an exception handler to io_bind_sockaddr.
Start listening on the socket. Record the display_number.
(server_x11_setup): Added argument single (and fail if it is
non-zero). Updated caller in server_session.c. Call
server_x11_setup. Ues the real display number when formatting the
DISPLAY string.

Rev: src/server_session.c:1.95
Rev: src/server_x11.c:1.10
Rev: src/server_x11.h:1.5