gdbusaddress, win32: don't rely on short names

win32 GDBus 1. Bug GTest This MR fixes issue #1566 (closed)

Dbus session process, executed with rundll libgio-2.0-0.dll was failing for paths containing spaces if short names were disabled.

The first commit that can be cherry-picked to 2.60 is changing the process execution to use related path .\libgio-2.0-0.dll with passing full path as a current directory.

Some problems discovered in the non-changed part of code doesn't show up as user-visible problems (yet?..) I only added code comments about them.

The next series of commits after is changing usage of rundll32.exe to gdbus.exe _win32_run_session_bus and adding gdbus.exe binary as a requirement for applications relying on dbus on win32.

The changed code was not covered by any tests, so a test is added. The test has some weaknesses due to problems in tested code unrelated to my changes - see test commit message for details. Still the test is useful for CI or manual testing.