deb: install a sample sogo.conf in /etc/sogo

While there, replace mkdir + cp combo with 'install'

packaging/debian-multiarch/sogo.postinst

@@ -20,6 +20,12 @@ case "$1" in
     configure)
           # update timestamp on imgs,css,js to let apache know the files changed
           find /usr/lib/GNUstep/SOGo/WebServerResources  -exec touch {} \;
+
+          # Enforce owner+mode on configuration
+          chmod 750 /etc/sogo
+          chown root:sogo /etc/sogo
+          chmod 640 /etc/sogo/sogo.conf
+          chown root:sogo /etc/sogo.conf
         ;;
 
     abort-upgrade|abort-remove|abort-deconfigure)