Changes in response to error report by S. Eken: GLR mode does not

Changes in response to error report by S. Eken: GLR mode does not
handle negative $ indices or $ indices in embedded rules correctly.
See <http://mail.gnu.org/archive/html/bug-bison/2003-01/msg00076.html>.

(b4_rhs_value): Change to use YYFILL macro.
(b4_rhs_location): Ditto.
(yyfill): New function to copy from stack tree into array
incrementally.
(yyuserAction): Modify to allow incremental move of semantic values
to rhs array when in GLR mode.
Define YYFILL to use in user-defined actions to fill semantic array
as needed.
Remove dummy use of yystack, as there is now a guaranteed use.
(yydoAction): Modify to allow incremental move of semantic values
to rhs array when in GLR mode.
(yyresolveAction): Ditto.
(yyglrShiftDefer): Update comment.
(yyresolveStates): Use X == NULL for pointers, not !X.
(yyglrReduce): Ditto.
(yydoAction): Ditto