Introducing new unix user and group 'xwiki' in debianpackages

Such a user and group should be the owner of the xwiki files. Adding other users to the group xwiki will grant them access to xwiki data.

So the package xwiki-enterprise-<application server>-common would be able to add the
user of the application server to the group xwiki (e.g. addgroup tomcat7
xwiki) for granting the application server access to the file /etc/xwiki/hibernate.cfg.xml.

A user/group based right management is very convenient for the system administrator and leads to fewer problems in general.