Make /opt/mono work as a prefix

This PR is just the stuff we discussed on Twitter. Adding these to the paths appears to work for that usage as a temporary measure.

In my (admittedly slightly unusual) usage of mono, I have several versions installed in e.g. `/opt/mono-3.2.8`, `/opt/mono-3.2.6`, `/opt/mono-SOMESHA1` etc, and have the currently "active" version symlinked to `/opt/mono`.