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.
Excellent question. It is supposed to read "su - -s /bin/sh", -s being an option for su to set an alternative shell. 

Sorry about that, I do not know when that got mangled in my commits. I actually do use a wrapper script with "-s" on a production server.