Handle Debian/Ubuntu packages places stylesheets in /etc.  Fixes missing CSS on RHEL

Really not sure why these are in /etc/, but that's now Debian does it.  Instead of linking from the correct /usr/share directory into /etc/nagios3 on source installs so the apache/nginx redirect works, just put the redirect only on debian package installs.  That's WAY cleaner and fixes missing CSS files on RHEL package installs.