Slapproxy now takes a mandatory 'root-path' argument.
When a local software url is a subpath of this root path, it is stored
as a relative path to the root path in the proxy database.

The urls are automatically expanded to their absolute path when read
from the datable, but they must be carefully adapted to be relative
at every place they are stored or compared to values in the database.

This makes it possible to seamlessly backup the slapproxy and restore
it on a different machine along with the local software releases.