- Use attach_md instead of hardcoding md(4) provider unit numbers

- Use attach_md instead of hardcoding md(4) provider unit numbers
- Implement a gmirror_test_cleanup function, which in turn calls
  geom_test_cleanup to clean up all md(4) providers allocated in the test
  run.
- Remove duplicate logic in test scripts for removing md(4) providers.
- Don't create files in /tmp (outside the kyua sandbox); use the current
  directory instead

MFC after: 3 weeks
Sponsored by: EMC / Isilon Storage Division