marsadm: use lamport clock as mtime of symbolic links

Fixes the following bug:
Symbolic links which are created in userspace get the current time
as mtime.
marsadm: correct wrong condition for checking exclusive access

The fixed bug was introduced in commit:

marsadm: 'create-resource' --force no longer checks for exclusive access
marsadm: command down includes disconnect

Fixes the following bug:
marsadm down does not disconnect the resource.