glx: use v2 of DRI_ConfigOptions

glx: use v2 of DRI_ConfigOptions

Most of the change is concerned with avoiding memory leaks, since v2 of
the DRI extension returns a malloc'ed string. This also allows us to
resolve the long-standing issue of keeping drivers loaded when returning
from glXGetDriverConfig.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>