ata problems _at_ r189170

HEAD _at_ r189170 fails to boot on a T41.  Two cd/dvd drivers are 
recognized (only 1 exists) and I get complaints of the form (and 
transcribed):

acd1: WARNING - ATAPI_IDENTIFY taskqueue timeout - completing request 
directory

(similarly for acd0) until there's a panic:

panic: mtx_lock() of destroy mutex _at_ .../../../kern_sema.c:79

(stack trace uninteresting).

    Sam