@@ typedef std::thread_cancel in cult::os::sched?

@@ maybe define USE_CXX_UNWIND or something in pthread.h
   so I can detect that?

@@ rename auto_lock to lock?

@@ maybe add sched::exception: os::exception i.e., "scheduling exception"?

@@ I am throwing exceptions in sched::thread that are not derived from
   sched::exception.