yacc.c: use negative numbers for errors in auxiliary functions

yyparse returns 0, 1, 2 since ages (accept, reject, memory exhausted).
Some of our auxiliary functions such as yy_lac and
yyreport_syntax_error also need to return error codes and also use 0,
1, 2.  Because it uses yy_lac, yyexpected_tokens also needs to return
"problem", "memory exhausted", but in case of success, it needs to
return the number of tokens, so it cannot use 1 and 2 as error code.
Currently it uses -1 and -2, which is later converted into 1 and 2 as
yacc.c expects it.

Let's simplify this and use consistently -1 and -2 for auxiliary
functions that are not exposed (or not yet exposed) to the user.  In
particular this will save the user from having to convert
yyexpected_tokens's -2 into yyreport_syntax_error's 2: both return -1
or -2.

* data/skeletons/yacc.c (yy_lac, yyreport_syntax_error)
(yy_lac_stack_realloc): Return -1, -2 for errors instead of 1, 2.
Adjust callers.
* examples/c/bistromathic/parse.y (yyreport_syntax_error): Do take
error codes into account.
Issue a syntax error message even if we ran out of memory.
* src/parse-gram.y, tests/local.at (yyreport_syntax_error): Adjust.

tests/local.at

@@ -632,8 +632,8 @@ yyreport_syntax_error (const yyparse_context_t *ctx]AT_PARAM_IF([, AT_PARSE_PARA
   YYUSE (\1);])])[]m4_bmatch(m4_defn([AT_PARSE_PARAMS]), [nerrs],[[
   ++global_nerrs;
   ++*nerrs;]])[
-  if (n == -2)
-    return 2;
+  if (n < 0)
+    return n;
   if (n)
   {]AT_LOCATION_IF([[
     LOCATION_PRINT (stderr, *yyparse_context_location (ctx));