MFC r303468 (by imp):
Move protocol specific stuff into a linker set object that's
per-protocol. This reduces the number scsi symbols references by
cam_xpt significantly, and eliminates all ata / nvme symbols. There's
still some NVME / ATA specific code for dealing with XPT_NVME_IO and
XPT_ATA_IO respectively, and a bunch of scsi-specific code, but this
is progress.