Refined the warning message for $, and $;

[Bug #16438]

io.c

@@ -7543,7 +7543,7 @@ rb_output_fs_setter(VALUE val, ID id, VALUE *var)
 {
     rb_str_setter(val, id, &val);
     if (!NIL_P(val)) {
-        rb_warn("non-nil $, will be deprecated");
+        rb_warn_deprecated("`$,'", NULL);
     }
     *var = val;
 }

string.c

@@ -10092,7 +10092,7 @@ rb_fs_setter(VALUE val, ID id, VALUE *var)
 		 rb_id2str(id));
     }
     if (!NIL_P(val)) {
-        rb_warn("non-nil $; will be deprecated");
+        rb_warn_deprecated("`$;'", NULL);
     }
     *var = val;
 }

test/ruby/test_string.rb

@@ -1771,7 +1771,7 @@ CODE
 
     begin
       fs = $;
-      assert_warn(/\$; will be deprecated/) {$; = " "}
+      assert_warn(/`\$;' is deprecated/) {$; = " "}
     ensure
       EnvUtil.suppress_warning {$; = fs}
     end