sd_bus_process — Drive the connection
sd_bus_process() drives the connection between the
message bus and the client. That is, it handles connecting,
authentication, and message processing. It should be called in a loop
until no further progress can be made or an error occurs.
sd_bus_process processes at most one incoming
message per call. If the parameter
r is not NULL
and the call processed a message,
*r is set to this message.
The caller owns a reference to this message and should call
when the message is no longer needed. If
r is not
NULL, progress was made, but no message was processed,
set to NULL.
If progress was made, a positive integer is returned. If no progress was made, 0 is returned. If an error occurs, a negative errno-style error code is returned.