Get rid of scsi_deinit(), and change scsi_init() back to a one-time

Get rid of scsi_deinit(), and change scsi_init() back to a one-time
initialization strategy, rather than pretending to do user reference
counting.  Previously, we would re-initialize the SCSI pool(9)s, which
had the fun consequence of causing sysctl(kern.pool.npools) to
infinite loop at IPL_VM.

ok krw@