refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

src/nvim/plines.c

@@ -233,7 +233,7 @@ int win_chartabsize(win_T *wp, char *p, colnr_T col)
   if (*p == TAB && (!wp->w_p_list || wp->w_p_lcs_chars.tab1)) {
     return tabstop_padding(col, buf->b_p_ts, buf->b_p_vts_array);
   } else {
-    return ptr2cells((char *)p);
+    return ptr2cells(p);
   }
 }