Setting up the environment for direct I/O (ATLASJT-398)

RunTier0Tests.logThis branch sets the environment variable that enables the fork handlers in xrootd for direct I/O, making the API fork safe. The variable is set in the shell that processes the input files. In this shell, the transform opens the input files to fetch general metadata from them (number of events, guid, etc.,), forking into a child process for each input file. If this xrootd variable isn't set, strange hangs occurr while trying to open more than one input file