[session] move context manager logic in the security context manager The `init_security` and `reset_security` method have no other user that the context manager itself. Having dedicated public function is not useful.

usr/lib/python2.4/site-packages/cubicweb/
etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances