no-lines: avoid leaving an empty line instead of the syncline

Currently, with --no-lines, instead of "#line file line\n", we emit
"\n".  Let's emit nothing.

* data/skeletons/bison.m4 (b4_syncline): Emit at end-of-line when enabled.
* data/skeletons/bison.m4, data/skeletons/c.m4, data/skeletons/glr.cc,
* data/skeletons/lalr1.cc, src/output.c: Use dnl after b4_syncline to
avoid spurious empty lines.

* tests/synclines.at (Sync Lines): Make sure that --no-lines is like
grep -v #line.
* tests/calc.at: Make sure that a rich grammar file behaves properly
with %no-lines.

data/skeletons/yacc.c

@@ -1699,7 +1699,7 @@ yyreduce:
   }]], [[
   switch (yyn)
     {
-      ]b4_user_actions[
+]b4_user_actions[
       default: break;
     }]])[
   /* User semantic actions sometimes alter yychar, and that requires