kes Separate read/write source strings to keep track of where

kes  Separate read/write source strings to keep track of where
     storage devices are used in a job.
kes  Implement a new method of keeping track of which storage
     device is used in a Job.  USTORE keeps both a pointer to the
     resource and to the source string.
kes  Modify all code to use new storage set subroutines in job.c
kes  Modify migrate.c so that the definitive selection of the storage
     resource is done in the do_migration_init() routine prior to
     the job going into the job queue.  This permits accurate
     deadlock detection (same read and write storage resource).
kes  Remove bfill from btape (I think it was a left over stub).

git-svn-id: https://bacula.svn.sourceforge.net/svnroot/bacula/trunk@3690 91ce42f0-d328-0410-95d8-f526ca767f89