__glXGetStringFromServer.  Other functions will likely be added in the
not too distant future.  Modify surrounding code to use
__glXGetStringFromServer.

As an aside, this fixes a long standing, dormant bug in
__glXInternalQueryServerString.  If memory could not be allocated to
hold the string retured by the server, _XEatData would be called with
'length'.  This is wrong.  'length' measures the size of the packet in
CARD32 units, but _XEatData expects the size to be in CARD8 units.