enhancement for in wrapper script in real-world deployments

Deploying eXist-db on a server usually involves setting up a special user that is the owner of the process. I found that if that user does not have a valid shell (/bin/false or /usr/sbin/nologin is not uncommon for daemon accounts), the wrapper script for starting the database fails (when using RUN_AS_USER).

This is a trivial patch that fixes the problem.
Hmmm, as it turns out, -s does not exist for "su" on macosx ......
that means we cannot use it?