additional return value for server listen functions

[Ralf Habacker]

Hi,

in dbus sources dbus_server_listen() handles some return values from the 
platform related listen function. For the autolaunch support on windows 
there is a new return code DBUS_SERVER_LISTEN_ADDRESS_ALREADY_USED 
required (see appended patch)

The question is if there is a better name for this return value ?

Regards
  Ralf