chainsaw-update: use a graph with branching in graph This will be relevant for the next improvement of `chainsaw-update`.

chainsaw-update: log actual locks breaking Previously, the command would simply state that it was about to break locks, not if there was actually some to break. This version is race-free. It would be also possible to display the content of the lock before hand (not race-free but informative in almost all cases).

vfs: have tryunlink tell what it did It is useful in certain circumstances to know whether vfs.tryunlink() actually removed something or not, be it for logging purposes.