[session] fix arguments default value and promote usage of security_enabled as session method. Closes #2481820 One should use session method rather than direct usage of the context manager of the same name. Fix default argument values for consistency with the context manager: when one omit an argument, meaning is "keep the current value", not "disable security".

#!/bin/sh -e
if [ "$1" = "purge" ] ; then
        update-rc.d cubicweb remove >/dev/null
fi
 
#DEBHELPER#
 
exit 0