gdbus does not support Windows SSPI authentication

Submitted by LRN @ruslanizhb

Link to original bug (#794206)

Description

GDBus only supports EXTERNAL, SHA1 and ANONYMOUS authentication mechanisms.

SHA1 depends on filesystem access rights being set up correctly (not sure whether it works on Windows). EXTERNAL depends on passing credentials as a side-metadata along with a byte sent over a UNIX socket, so this is not doable on Windows (dbus has a hacky way of achieving something similar, but hardly worth copying). ANONYMOUS is...anonymous.

Therefore, i propose to use SSPI to perform authentication.