- Continue implementing migration.

- Implement support for removable filesystems in SD.
- Ensure that btraceback scripts can be read by anyone.
- Replace dvd-freespace and dvd-writepart by dvd-handler.
- Correct bug where canceling restore before the FD contacts
  the SD causes the drive to be left in read mode.
- Move ofline_or_rewind into DEVICE::close().
- Eliminate close_device.
- Convert several dev subroutines to methods (e.g. bsf,
  eod, ...)
- Eliminate force_close_device().
- Implement Device Type directive in Device resource that
  can have values File, Tape, Fifo, DVD, or Prog.
- Add has_cap() method to Device.