OS X port of RioFS

The commits add support for building and running RioFS on OS X. All commits but c37543b69dff01e863ad58d385965d63d1582695 should not affect RioFS' behavior on Linux.

In commit c37543b69dff01e863ad58d385965d63d1582695 volumes are unmounted before exiting the event loop. This is very important on OS X and should have no ill effects on Linux, but needs to be tested on Linux.