preserve the indentation in the ouput

Preserve the actions' initial indentation.  For instance, on

    | %define api.value.type {int}
    | %%
    | exp: exp '/' exp { if ($3)
    |                     $$ = $1 + $3;
    |                   else
    |                     $$ = 0; }

we used to generate

    |     { if (yyvsp[0])
    |                     yyval = yyvsp[-2] + yyvsp[0];
    |                   else
    |                    yyval = 0; }

now we produce

    |                  { if (yyvsp[0])
    |                     yyval = yyvsp[-2] + yyvsp[0];
    |                   else
    |                     yyval = 0; }

See https://lists.gnu.org/archive/html/bison-patches/2019-06/msg00012.html.

* data/skeletons/bison.m4 (b4_symbol_action): Output the code in
column 0, leave indentation matters to the C code.
* src/output.c (user_actions_output): Preserve the incoming
indentation in the output.
(prepare_symbol_definitions): Likewise for %printer/%destructor.
* tests/synclines.at (Output columns): New.

data/skeletons/bison.m4

@@ -449,7 +449,7 @@ m4_define([b4_symbol_action],
                    [(*yylocationp)])dnl
     _b4_symbol_case([$1])[]dnl
 b4_syncline([b4_symbol([$1], [$2_line])], [b4_symbol([$1], [$2_file])])dnl
-      b4_symbol([$1], [$2])
+b4_symbol([$1], [$2])
 b4_syncline([@oline@], [@ofile@])dnl
         break;