ensure session_key is set

cherry-picked from Evolvis
Conflicts:
	src/common/include/session.php
need {core/session_key} consistently as www-data *and* root

rest from Evolvis; e64f1d24365cf4dfcef12d3227eb6a6d3d14fc35
already did the gros (of course, admins must update their
/etc/gforge/httpd.conf.d/secrets.inc from their secret key
and /usr/share/gforge/etc/httpd.conf.d-fhs/secrets.inc by
themselves)
Removed empty and useless -mta-courier package
Run Lintian on generated packages
Merged from 5.2