Use /etc/mono/ (or equivalent MONO_CFG_DIR) for cert store, on Unix

On every major Linux distribution, the system cert store lives in /etc, and the Java cert store lives in /etc - it certainly doesn't belong in /usr/share, which should only be for static app-provided data. Mono's /usr/share/.mono/ is a major FHS violation, albeit it's much easier to retrieve as a location than the alternative I'm using here.

This patch includes a simple migration behaviour to move any old system certs to the new location.