Don't just support bup commands; allow arbitrary commands as arguments,
i.e. instead of
bup mux BUP_SUBCOMMAND ...
support
bup mux COMMAND ...
i.e.
bup mux tar cvf ...
Reviewed-by: Gabriel Filion <gabster@lelutin.ca>
[rlb@defaultvalue.org: rebased to master; removed not yet applicable
on--server-cmd.py changes] Signed-off-by: Rob Browning <rlb@defaultvalue.org> Tested-by: Rob Browning <rlb@defaultvalue.org>