Change the handling of @s so that they behave exactly like $s.

Change the handling of @s so that they behave exactly like $s.
There is now a pseudo variable @$ (readble and writable), location
of the lhs of the rule (by default ranging from the location of
the first symbol of the rhs, to the location of the last symbol,
or, if the rhs is empty, YYLLOC).

* src/bison.s1 [YYLSP_NEEDED] (yyloc): New variable, twin of
yyval.
(yyparse): When providing a default semantic action, provide a
default location action.
(after the $): No longer change `*YYLSP', just stack YYLOC the
same way you stack YYVAL.
* src/reader.c (read_declarations): Use warns.
(copy_guard, case '@'): Also recognize `@$', expanded as `YYLOC'.
(copy_action, case '@'): Likewise.
Use a standard error message, to save useless work from
translators.