The idle thread is now actually run when the system is idle because it
truly goes idle. The idle thread is made power efficient by using the hlt
instruction rather than a busy loop.

The new futex(2) system call is used to implement fast user-space mutexes,
condition variables, and semaphores. The same backend and design is used as
kutexes for truly sleeping kernel mutexes and condition variables.

The new exit_thread(2) flag EXIT_THREAD_FUTEX_WAKE wakes a futex.

Sleeping on clocks in the kernel now uses timers for true sleep.

The interrupt worker thread now truly sleeps when idle.

Kernel threads are now named.

This is a compatible ABI change.