Save and restore the js.exec_path, exec_dir, and exec_file properties

When invoking a nested JS script, these properties of the "js" object would
be overwritten and not restored, as discovered/reported by Nightfox when his
trivial game script would indirectly execute yesnobar.js, his subsequent use
of js.exec_dir would point to the wrong location (the "exec" directory, parent
of yesnobar.js, and not the direcctory where his game script was located).
The exec_path and exec_file properties had the same problem as demonstrated
by a simple test.js placed in (and executed from) a directory other than the
"exec" dir:
	function f() {
        print("js.exec_path = " + js.exec_path);
        print("js.exec_dir = " + js.exec_dir);
        print("Js.exec_file = " + js.exec_file);
	}
	f();
	console.yesno("test");
	f();

This would only trigger the problem when executed from the BBS and whebn the
YesNoQuestion text.dat string invokes the "yesnobar" module via EXEC @-code and
yesnobar.js exists (in exec or mods dir), superceding yesnobar.bin which does
not trigger this issue (not a JavaScript mod).