Calling bup-server with "-r host:" fails to activate dumb mode.

Calling bup-server with "-r host:" fails to activate dumb mode.

When no remote directory is supplied to the -r option, no set-dir
command is sent to the server. This has the weird side effect that the
server actually does not check whether it needs to be in "smart" or
"dumb" mode.

By forcing all commands to make that verification, we'll ensure that the
server mode is correct.

Signed-off-by: Yung-Chin Oei <yungchin@yungchin.nl>
Reviewed-by: Gabriel Filion <lelutin@gmail.com>
Reviewed-by: Zoran Zaric <zz@zoranzaric.de>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>