Commit adaptive mutexes. This seems to have introduced some new
crashes but it's not clear to me yet if these are a problem with
the mutex implementation or ZFSs usage of it.

Minor taskq fixes to add new tasks to the end of the pending list.

Minor enhansements to the debug infrastructure.

git-svn-id: https://outreach.scidac.gov/svn/spl/trunk@94 7e1ea52c-4ff2-0310-8f11-9dd32ca42a1c