Fix neovim tag bugs uncovered by vim-8.2.0088,

but not related to the patch.  Specifically:

    - settagstack()'s e_listreq is in the wrong place
    - in :ltag, vim_strncpy -> xstrlcpy length parameter is different
        xstrlcpy's length includes the null terminator (so add one)
    - in :ltag, STRNCAT -> xstrlcat takes dest size, not number to copy
        use snprintf instead

src/nvim/eval/funcs.c

@@ -8809,8 +8809,6 @@ static void f_settagstack(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 
     if (set_tagstack(wp, d, action) == OK) {
       rettv->vval.v_number = 0;
-    } else {
-      EMSG(_(e_listreq));
     }
 }