This is a bit of breaking change:
After this commit the apps relying of win32 dbus autolaunching,
need to install gdbus.exe alongside with libgio-2.0-0.dll.

A new command for gdbus tool is used for running server:
gdbus.exe _win32_run_session_bus

To implement it gdbus.exe uses the same exported function
g_win32_run_session_bus that earlier was used by rundll.
So (private) ABI was not changed.

It runs the bus syncronously, exiting after inactivity timeout -
all exactly like it was runed earlier with the help of rundll32.

While private exported function may have _some_
version compatibility issues between gdbus.exe and libgio-2.0-0.dll
compiling dbus server registration logic directly into gdbus.exe
can lead to _more hidden and more complex_ compatibility issues
since the names and behaviour of syncronization objects
used to publish server address would be required compatible between
gdbus.exe and libgio-2.0-0.dll.

So using "private" expor...