mux: support wrapping arbitrary commands

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>