These need special casing in the publisher to be treated as successful
and result in transaction commits. Also prefer `raise Redirect` style
over `REQUEST.RESPONSE.redirect`.

Opened database are now found in `Zope2.opened` next to `Zope2.DB`.
Also stop setting client/instance home variables onto `__builtin__`,
those are available from `App.config.getConfiguration()` as
`instancehome` and `clienthome` attributes.

Based on zopefoundation/five.pt@197e65819013fbfd3a436286737f20ddc4c48813.