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.
It would be possible to use `sudo` instead of `su` - that works on Linux and OS X, but I am not sure if this might have any further ramifications on some of the supported platforms. Another way would be to use `-s` only on linux platforms, but that would not solve the issue on the other platforms.