slapproxy: Fix software URL migration

The `local-software-release-url` option allows migrating the software
URLs which are local paths by rebasing them on the path provided by
the option.

Do not migrate software release URLs if the old root path and the
new root path are subpaths or superpaths one of the other.

In addition, do not migrate an URL if the old one refers to an
existing file and the new one doesn't.

Also, create a backup of the database before migrating.

See merge request nexedi/slapos.core!338