DECC$EFS_CASE_PRESERVE

   With DECC$EFS_CASE_PRESERVE enabled, case is preserved for
   filenames on ODS-5 disks.

   With DECC$EFS_CASE_PRESERVE disabled, UNIX style filenames are
   always reported in lowercase.

   However, note that enabling DECC$EFS_CASE_SPECIAL overrides the
   setting for DECC$EFS_CASE_PRESERVE.