sched: fix double-free of detached threads

The detached thread reaper deletes zombies, but our pthread implementation
also deletes dead pthreads (using the container object).

Fix by making the base thread use the set_cleanup() method to set up a
deleter, which is then overridden by pthreads.

core/sched.cc

@@ -295,6 +295,12 @@ thread::thread(std::function<void ()> func, attr attr, bool main)
     });
     setup_tcb();
     init_stack();
+    if (_attr.detached) {
+        // assumes detached threads directly on heap, not as member.
+        // if untrue, or need a special deleter, the user must call
+        // set_cleanup() with whatever cleanup needs to be done.
+        set_cleanup([=] { delete this; });
+    }
     if (main) {
         _vruntime = 0; // simulate the first schedule into this thread
         _status.store(status::running);
@@ -620,7 +626,6 @@ void thread::reaper::reap()
                 auto z = _zombies.front();
                 _zombies.pop_front();
                 z->join();
-                delete z;
             }
         });
     }