Add render-once support
Allows, for example, generating zope's inituser file using templates.
On the specific topic of zope's inituser file, there is a little-known through-the-web solution which makes zope ignore the inituser file: just create any other use in zope root's acl_users. This would allow much simpler instanciation, but risks exposing and/or locking users out of any old instance upgrading to a Software Release version which always creates this file.
I need this to be released to upgrade ERP5 SR, and @yusei is waiting on my SR upgrade for a deployment, so we would appreciate if the review of this (rather short) change could be quick.