MacRelixLib:  Fix destructive OS thread assignment

This didn't exhibit a bug in practice, since we'd only assigned to empty
os_thread_box objects.
MacRelixLib:  Simplify os_thread_box destructor
relix:  Avoid intrusive_ptr in os_thread_box ctor

This removes all use of boost::intrusive_ptr from os_thread_box's member
function declarations.
relix:  Use raw vs. intrusive_ptr in os_thread_box

Insulate os_thread_box users from details of shared reference management
by hiding use of intrusive_ptr in the corpus file.
Merge branch 'unintrusive'