attend error paths, more strict asserts and code consistency

- atastart() and ata_kill_pending() now KASSERT() that all xfers on queue
  have same channel
- inactive xfers are killed via new reason KILL_GONE_INACTIVE, controller
  code must not call any resource deactivation in that case
- c_intr() must call ata_waitdrain_xfer_check() as first thing, and must not
  further touch any xfer structures on exit path; any resource cleanup
  is supposed to be done in c_kill_xfer()
- c_kill_xfer() should never call atastart()
- ata_waitdrain_check() removed, replaced by ata_waitdrain_xfer_check()
- ATA_DRIVE_WAITDRAIN handling converted to use condvar
- removed unused ata_c callback

(jdolecek)