convert the iodone_lock to a mutex, and use a condvar for signalling.

this only handles the smallest use of old simple_lock/tsleep/wakeup
APIs inside raidframe, and it points out that cv(9)'s have only one
wait channel per cv, whereas each tsleep() caller can specify a
different wait channel.  this change removes the difference between
normal raidio and waiting for IO during shutdown.

i've tested this one 3 systems, ran atf, and had mlelstv and rmind
review the change.

(mrg)