- Fix a potential source of deadlocks due to the usage of vnsprintf
   - Fixes for permission related failures during initialization of dataset managers:
     - Add the possibility to define the dataset information sources via the
       directive 'xpd.datasetsrc'. In this way the permissions should be
       set correctly and the related problems disappear.
     - In TProofDataSetManagerFile, switch in browse mode (instead of failing
       construction) if the write permissions are not present.


git-svn-id: http://root.cern.ch/svn/root/trunk@28590 27541ba8-7e3a-0410-8455-c3a389f83636