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.

There are so many import dependencies between OFS and webdav, it's
otherwise too easy to accidentally cause a cycle and silently break
one module.