Fix Solaris thread dependency by using pthreads

This is a portability change which removes the dependence of the Solaris
thread library. All locations where Solaris thread API was used before
have been replaced with equivilant Solaris kernel style thread calls.
In user space the kernel style threading API is implemented in term of
the portable pthreads library. This includes all threads, mutexs,
condition variables, reader/writer locks, and taskqs.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>