Add a detached thread class (with a private destructor, so it can only be
allocated on the heap).  The implementation uses a reaper thread to wait
until the detached thread is dead, and then ->join() and delete it.

fixes loading unit tests with "main" as the entry point.

This is like printf() but prints straight to the console instead of going
through the stdio library, the VFS and the device layer.

To be used for debugging outout in the kernel.

* 'master' of github.com:cloudius-systems/osv:
  interrupt manager: use range-based for loop instead of manually looping
  sched: optimize scanning the incoming_wakeups array
  sched: add an atomic cpu_set type

of the pci virtio config size to a function using the first.

Protect the array with a cpu_set (bit set -> corresponding queue non-empty).
This means we need to scan one word, instead of nr_cpus (plus of course all
the cpus for which an incoming is waiting).

Supporting up to 64 cpus for now.