egl_xdri steals code from src/glx/x11/. This makes it broken from time
to time when there is a bigger change to the GLX code. As egl_dri2 has
been merged, which also functions as a DRI2 driver loader, remove
egl_xdri for good.
EGL_DRIVERS_PATH gives a list of colon-separated directories. The given
directories will be searched when preloading drivers.
This is based on Mike Stroyan's patch, which honors the variable in
_eglPreloadDisplayDrivers. It is extended to honor the variable also in
_eglPreloadUserDriver and _eglPreloadDefaultDriver in this version.