go/zodb/fs1: Report URL with file:// schema included

In ZODB/go when there is no schema in zurl, open automatically prepends
file:// . However filename itself could contain ":" and so generally
speaking it is incorrect to return URL without file:// schema prepended
to file name.

Another reason to always use fully-constructed URLs with schema, is
interoperability with ZODB/py - there zodburi, when given zurl without
schema, does not make any assumption that it is of file:// kind and
rejects opening such URIs.

go/zodb/storage/fs1/filestorage.go

@@ -135,7 +135,7 @@ func (fs *FileStorage) LastOid(_ context.Context) (zodb.Oid, error) {
 }
 
 func (fs *FileStorage) URL() string {
-	return fs.file.Name()
+	return "file://" + fs.file.Name()
 }
 
 // freelist(DataHeader)