c: don't emit an error message when the scanner returns YYERRCODE

* data/skeletons/yacc.c (yyparse): When the scanner returns YYERRCODE,
go directly to error recovery (yyerrlab1).
However, don't keep the error token as lookahead, that token is too
special.
* data/skeletons/lalr1.cc: Likewise.

* examples/c/bistromathic/parse.y (yylex): Use that feature to report
nicely invalid characters.
* examples/c/bistromathic/bistromathic.test: Check that.
* examples/test: Neutralize gratuitous differences such as rule
position.

* tests/calc.at: Check that case in C only.
The other case seem to be working, but that's an illusion that the
next commit will address (in fact, they can enter endless loops, and
report the error several times anyway).

examples/test

@@ -89,11 +89,13 @@ run ()
   shift
 
   # Expected output.
-  if $strip_prompt; then
-      $echo "$1" | sed -e '/^> /d' >exp
-  else
-      $echo "$1" >exp
-  fi
+  $echo "$1" |
+    sed -e 's/Reducing stack by rule .* (line .*):/Reducing stack by rule XX (line XXX):/g' |
+    if $strip_prompt; then
+      sed -e '/^> /d'
+    else
+      cat
+    fi >exp
   shift
 
   # Effective exit status.
@@ -102,7 +104,14 @@ run ()
   prog "$@" - <input >out_eff 2>err_eff || sta_eff=$?
 
   # Combine effective output and error streams.
-  { cat out_eff && $noerr || sed -e 's/^/err: /g' err_eff; } >eff
+  {
+    cat out_eff
+    if ! $noerr; then
+       sed -e 's/^/err: /g' \
+           -e 's/Reducing stack by rule .* (line .*):/Reducing stack by rule XX (line XXX):/g' \
+           err_eff
+    fi
+  } >eff
 
   if test $sta_eff -eq $sta_exp; then
     if cmp eff exp 2>/dev/null; then