runtime/qio: use getcwd instead of getwd

getwd is deprecated in favor of getcwd.

Related to #325.
Using extern var MAXPATHLEN would get it for you in the Chapel file, but in asking around I was reminded that this value is going to differ depending on the operating system so it'll be an annoying test to have around.  So we'll just leave the test system as is, see if you can make the MAXPATHLEN case for our own edification but don't put too much into it as we doubt it'll be encountered often.

Thanks again for handling this!