Mutex tests updated to use task queues instead of work queues.

Mutex tests updated to use task queues instead of work queues.

Mainly for portability reasons I have rebased the mutex tests on Solaris
taskqs instead of linux work queues.  The linux workqueue API changed post
2.6.18 kernels and using task queues avoids having to conditionally detect
which workqueue API to use.

Additionally, this is basically free additional testing for the task queues.
Much to my surprise after updating these test cases they did expose a long
standing bug in the taskq_wait() implementation.  This patch does not
address that issue but the followup patch does.