Abstract the memory allocation, scsibus_softc data copying and

Abstract the memory allocation, scsibus_softc data copying and
flag setting bits of creating a scsi_link into scsi_alloc_link().

Shrinks the bloated scsi_probe_link() a bit, makes it possible to
eventually create a useable scsi_link even when scsi_probe_link()
can't attach a device.

Developed from part of a diff submitted by Scott Nicholas via
tech@.