Add in the threaded async open engine.

Add in the threaded async open engine.

Fixes all issues raised originally. This code
will only do threaded opens with thread-specific
credentials (Linux for now) and changes credentials
before doing the call. Also only fires on O_CREAT|O_EXCL
so will only create new files, never open old ones
async.

Volker, this is isolated enough that it shouldn't
prevent you from refactoring it into a new module
when the aio pread/pwrite code is moved into the
default aio path.

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Jul 11 08:04:56 CEST 2012 on sn-devel-104