Debian package : easier way to change user renderd is run with

This is a patch provided by sletuffe on github:
https://github.com/openstreetmap/mod_tile/pull/1




git-svn-id: http://svn.openstreetmap.org/applications/utils/mod_tile@29419 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4