Send command output via sideband.

Send command output via sideband.

The next step towards sideband connections is to send the output of
each command as a sideband packet. To this aim, afs_cb_result_handler(),
pass_buffer_as_shm() and the callback_result_handler typedef are modified
to receive an additional "band" parameter.

For regular command output the value of this parameter is SBD_OUTPUT,
the sideband designator for normal output. The client code is patched
to add the contents of received SBD_OUTPUT sidband packets to the
buffer tree for the normal ouput.

However, log messages, which are only written to the log output
of the server at the moment, may also passed to the client via the
SBD_XXX_LOG designators, where XXX is the name of one of the usual
log levels for para_log().

The patch contains quite a few changes of the form

	if (use_sideband)
		do_new_stuff()
	else
		do_it_as_before()

which is a bit ugly, but we may remove the old method in 0.5.0,
which will support only sideband connections.