patch 8.0.0517: there is no way to remove quickfix lists

Problem:    There is no way to remove quickfix lists (for testing).
Solution:   Add the 'f' action to setqflist(). Add tests. (Yegappan
            Lakshmanan)

src/evalfunc.c

@@ -10038,7 +10038,8 @@ set_qf_ll_list(
 	    act = get_tv_string_chk(action_arg);
 	    if (act == NULL)
 		return;		/* type error; errmsg already given */
-	    if ((*act == 'a' || *act == 'r' || *act == ' ') && act[1] == NUL)
+	    if ((*act == 'a' || *act == 'r' || *act == ' ' || *act == 'f') &&
+		    act[1] == NUL)
 		action = *act;
 	    else
 		EMSG2(_(e_invact), act);