Bug 1309827

Description of problem:
When starting xpra without explicitly installing dbus-x11, it will show a lot of messages like these:
```
2016-02-18 18:54:18,722 Warning: menu forwarding is disabled:
2016-02-18 18:54:18,722  cannot load dbus helper: org.freedesktop.DBus.Error.Spawn.ExecFailed: /usr/bin/dbus-launch terminated abnormally without any error message
[...]
2016-02-18 18:54:20,206 Warning: failed to load or register our dbus notifications forwarder:
2016-02-18 18:54:20,206  org.freedesktop.DBus.Error.Spawn.ExecFailed: /usr/bin/dbus-launch terminated abnormally without any error message
2016-02-18 18:54:20,206  if you do not have a dedicated dbus session for this xpra instance,
2016-02-18 18:54:20,206  use the 'notifications=no' option
2016-02-18 18:54:20,208 cannot load dbus helper: org.freedesktop.DBus.Error.Spawn.ExecFailed: /usr/bin/dbus-launch terminated abnormally without any error message
2016-02-18 18:54:20,212 Error setting up our dbus server:
2016-02-18 18:54:20,212  org.freedesktop.DBus.Error.Spawn.ExecFailed: /usr/bin/dbus-launch terminated abnormally without any error message
[...]
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
E: [pulseaudio] core-util.c: Failed to connect to system bus: Failed to connect to socket /run/dbus/system_bus_socket: No such file or directory
```

Version-Release number of selected component (if applicable):
0.16.1-2.fc24

How reproducible:
Every time

Steps to Reproduce:
1. Install xpra on a system without dbus-x11 (thus without dbus-launch)

Actual results:
s.a.

Expected results:
No error and warning messages

Additional info: