* dbusbind.c: Fix several errors and compiler warnings. Reported

* dbusbind.c: Fix several errors and compiler warnings.  Reported
by Tom Tromey <tromey@redhat.com>
(XD_ERROR, XD_DEBUG_MESSAGE)
(XD_DEBUG_VALID_LISP_OBJECT_P): Wrap code with "do ... while (0)".
(xd_append_arg): Part for basic D-Bus types rewitten.
(xd_retrieve_arg): Split implementation of DBUS_TYPE_BYTE and
DBUS_TYPE_(U)INT16.  Don't call XD_DEBUG_MESSAGE with "%f" if not
appropriate.
(xd_read_message): Return Qnil.  Don't signal an error; it is not
useful during event reading.
(Fdbus_register_signal): Signal an error if the check for
FUNCTIONP fails.
(Fdbus_register_method): New function.  The implementation is not
complete, the call of the function signals an error therefore.
(Fdbus_unregister_object): New function, renamed from
Fdbus_unregister_signal.  The initial check signals an error, if
it the objct is not well formed.