Pass command via sideband.

Pass command via sideband.

This makes para_cclient pass the paraslash command and its arguments
as a sideband packet to para_server if possible.

For sideband connections the command and its arguments are stored as
NULL-terminated strings. This is better than separating by newlines
(as we do for non-sideband connections) because it allows for arguments
containing newlines.

Suitable helpers for creating and parsing a buffer of NULL-terminated
strings, are provided in client_common.c and command.c, respectively.

No change for non-sideband connections.