Better js_dumpstack code

[mulle-nat]

To get a grip on the BOT/TOP STACK stuff with the negative or positive indices I needed more readable output, like this:

STACK 0 (-6)          [CFunction Object]
      1 (-5)          [Script [string]]
BOT>  2 (-4)          undefined
      3 (-3/+1)       [CFunction print]
      4 (-2/+2)       undefined
      5 (-1/+3)       'hello: VfL Bochum 1848'
TOP>  6

This function does this:

static void js_dumpstack_fp(js_State *J, FILE *fp)
{
   int   i;
   int   n;

   for( i = 0; i < TOP; ++i)
   {
      fprintf( fp, i == BOT ? "BOT>  " : (i == 0 ? "STACK " : "      "));
      n = fprintf( fp, "%d (-%d", i, TOP - i);
      if( i > BOT)
         n += fprintf( fp, "/+%d", i - BOT);
      n += fprintf( fp, ")");
      while( n < 16)
      {
         fputc( ' ', fp);
         ++n;
      }
      js_dumpvalue_fp(J, STACK[i], fp);
      fputc('\n', fp);
    }
   fprintf( fp, "TOP>  %d\n", TOP);
}

static void js_dumpstack(js_State *J)
{
   js_dumpstack_fp( J, stderr);
}