kes Rework a lot of subroutines in dev.c to take dcr as an

kes  Rework a lot of subroutines in dev.c to take dcr as an
     argument. This is done to eliminate the usage of attached_dcrs
     in lseek().
kes  Change truncated_dvd to blank_dvd, which seems more suitable.
kes  Apply most of Richard Mortimer's truncate patch.
kes  Create lseek() method for DEVICE that takes dcr as an
     argument. This is to eliminate the use of attached_dcrs in
     lseek().  The calls to lseek_dev() must still be changed.

git-svn-id: https://bacula.svn.sourceforge.net/svnroot/bacula/trunk@3468 91ce42f0-d328-0410-95d8-f526ca767f89