Enable GOTCHA via LD_PRELOAD

[adammoody]

To enable pre-built applications to run without needing to recompile or relink, it would be nice to have LD_PRELOAD support. Such apps will not call unifycr_mount/unmount, so this library will need to initialize another way. We could perhaps intercept /unifycr by default, and we could read an environment variable to acquire the desired mount point if the user desires something different.

An example of how to LD_PRELOAD with GOTCHA is available here:
https://github.com/LLNL/GOTCHA-tracer