Add locking to the bt(4) driver and mark it MPSAFE.

Add locking to the bt(4) driver and mark it MPSAFE.
- Use device_printf() and device_get_unit() instead of storing the unit
  number in the softc.
- Remove use of explicit bus space handles and tags.
- Return an errno value from bt_eisa_attach() if an error occurs rather
  than -1.
- Use BUS_PROBE_DEFAULT rather than 0.

Tested by:	no one