Do not create paths starting "//" when root is just "/"

(cherry picked from commit bd8b3be0c7535e74ca8b63969be5c2bece0d3a3b)
Conflicts:
	configmgr/source/access.cxx

Change-Id: If0b413a4fdd93465074548c7ea5451288c1d12aa
Reviewed-on: https://gerrit.libreoffice.org/6961
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>