Rediffed version, with Mike's isp fix and taking the new
scsi_add_host users in usb in account.

Currently this is juist a new name for scsi_register, but we make
sure new-style drivers never call scsi_register/scsi_unregister
but always scsi_host_alloc/scsi_host_put in this patch so the
next patch can introduce code specific to legacy drivers in
the former.  Also cleanup scsi_register/scsi_host_alloc a bit.