Even though GetStartupInfo() in g_win32_run_session_bus() would
tell us that STARTF_FORCEONFEEDBACK flag is not set, it still
affects the rundll32 process for some reason.

This means that Windows WM changes mouse cursor to IDC_APPSTARTING for
a few seconds when rundll32 runs g_win32_run_session_bus(). Since
g_win32_run_session_bus() never satisfies the conditions set by
STARTF_FORCEONFEEDBACK, the busy cursor only goes away after a
timeout.

Fix this by explicitly running GetMessage(). To ensure that GetMessage()
doesn't block, post a quit message immediately before calling it.

https://bugzilla.gnome.org/show_bug.cgi?id=760694