xgl: update xglDbg.h for XGL_INSTANCE

xgl: update xglDbg.h for XGL_INSTANCE

Add XGL_INSTANCE as the first parameter of xglDbgRegisterMsgCallback(),
xglDbgUnregisterMsgCallback(), and xglDbgSetGlobalOption().

The loader is simplified.  loader_log() is modified to always output to
stderr.

glave is modified to register its message callback after xglEnumerateGpus(),
and xglDbgUnregisterMsgCallback() is called only before xglDestroyInstance().

XGL_INSTANCE is ignored by the layers and the driver for now.

Bump up API version to XGL_MAKE_VERSION(0, 56, 1).