At the same time implement synchronous wrappers of the POSIX functions.
These will be undocumented until we settle on an API. Works like this
// returns promise as before
posix.mkdir("test").addCallback(function () {
sys.puts("done");
});
// returns undefined, executed synchronously.
posix.mkdirSync("test");
sys.puts("done");
This refactoring is a step towards allowing promises to be implemented
purely in javascript.
