- Fix mode change open in btape.c

- Use nonblocking opens on Linux only.
- Move set_blocking code for tapes into method to
  simplify main line code.
- Cleanup the code for ensuring we advance tape on fsf.
  This should fix IBM problems -- patch from Adam Thorton
  was very helpful.
- Remove test for BMT_EOD in fixup_device... this eliminates
  need for status_dev() routine.
- Before doing label ensure device is in read-write mode.