loader: Factor out the common driver opening logic from each loader.

loader: Factor out the common driver opening logic from each loader.

I copied the code from egl_dri2.c, but the functionality was equivalent
between all the loaders other than their particular environment variables.

v2: Drop the logging function equivalent to loader_default_logger()
    (requested by Eric, Emil).  Move the SCons workaround across.  Drop
    the now-unused driGetDriverExtensions() declaration that was lost in a
    rebase.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com> (v1)
Reviewed-by: Emil Velikov <emil.velikov@collabora.com> (v1)