address low-hanging fruit in port to libuv

As noted in #3, os_unix.c will eventually be ported to libuv. Grab the
low-hanginf fuit for that task by using the exact like-for-like functionality
provided by libuv which can be used to replace some code in os_unix.c.
Specifically ``mch_chdir()`` and ``mch_total_mem()``.

The longest journey begins with the smallest step :).