Fix --spice-debug
Fix --spice-debug

If spice_util_set_debug() gets called before spice_util_get_debug(),
then the value set using spice_util_set_debug() will be overridden
by the result of g_getenv("SPICE_DEBUG") != NULL the first time
spice_util_get_debug() is called.

This causes remote-viewer --spice-debug to not enable debug as
advertised if SPICE_DEBUG is not set.

An alternate fix would have been to set debug_once.status to
G_ONCE_STATUS_READY but then we would lose the thread-safety
guarantees GOnce gives us.

freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,elmarco/spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk