Bug 253

System fails to boot when using the latest version of the Adaptec 2842
drive (sys/i386/scsi/aic7xxx.c).  About 15 - 20 "sd1: Target Busy" 
messages are displayed on the console, and the system panics because
it is unable to mount the root file system.  

I tracked this down to the fact that the driver is now enabling "tagged
queuing" for the ST31230 SCSI-2 disk.  If I disable the line of code
that does this, the system boots fine.

Fix: A workaround is to disable tagged queuing for now:
How-To-Repeat: 
Boot with a device that supports tagged queuing (or one that the
driver thinks does).

Here is all of the relevant information from the boot sequence after
I disabled the tagged queing code and with AHC_DEBUG turned on.

ahc0: scb 2104 bytes; SCB_SIZE 19 bytes, ahc_dma 8 bytes
ahc0: reading board settings
ahc0: 284x Single Channel, SCSI Id=7, aic7770 <= Rev C, 4 SCBs
NEEDSDTR == 0xfffa
NEEDWDTR == 0x0
ahc0: Downloading Sequencer Program...Done
ahc0 at 0x1000-0x10ff irq 11 on eisa slot 1
ahc0: Probing channel A
scbus0: ahc0 waiting for scsi devices to settle
ahc0: target 0, lun 0 (probe0) requests Check Status
ahc0: target 0, lun 0 (probe0) Sending Sense
(ahc0:0:0): "CDC ST1239NS 9212" is a type 0 fixed SCSI 1
sd0(ahc0:0:0): Direct-Access 
sd0(ahc0:0:0): 194MB (398790 sectors), 1268 C 9 H 34 S/T 512 B/S
ahc0: target 1 synchronous at 10.0MB/s, offset = 0xf
ahc0: target 1, lun 0 (probe0) requests Check Status
ahc0: target 1, lun 0 (probe0) Sending Sense
ahc0: target 1 Tagged Queuing Device
(ahc0:1:0): "SEAGATE ST31230N 0290" is a type 0 fixed SCSI 2
sd1(ahc0:1:0): Direct-Access 
sd1(ahc0:1:0): 1010MB (2069860 sectors), 3992 C 5 H 103 S/T 512 B/S
ahc0: target 2, lun 0 (probe0) requests Check Status
ahc0: target 2, lun 0 (probe0) Sending Sense
(ahc0:2:0): "ARCHIVE VIPER 150  21944 -001" is a type 1 removable SCSI 1
st0(ahc0:2:0): Sequential-Access st0: Archive  Viper 150 is a known rogue
density code 0x0, ahc0: target 2, lun 0 (st0) requests Check Status
ahc0: target 2, lun 0 (st0) Sending Sense
 drive empty
ahc0: target 1, lun 0 (sd1) requests Check Status
ahc0: target 1, lun 0 (sd1) Sending Sense
ahc0: target 1, lun 0 (sd1) requests Check Status
ahc0: target 1, lun 0 (sd1) Sending Sense
...