0.9.2.9: thread objects

0.9.2.9: thread objects
  * Public interface changes
    ** proper thread objects instead of thread ids
    ** (MAKE-THREAD FN &KEY NAME) => THREAD
    ** (THREAD-NAME THREAD): threads have names (useful for debugging,
    logging)
    ** (THREAD-ALIVE-P THREAD)
    ** *CURRENT-THREAD* special is bound in each thread
    ** (LIST-ALL-THREADS) returns a list of all active threads
  * Notes
    ** thread-init moved earlier in cold-init and reinit
    ** the lisp side does not ever use os_thread_t (it was problematic due
       to pthread_t being opaque) but struct thread *
    ** threads are reaped (i.e. the thread is pthread_joined and struct
       thread* is freed) by the thread object's finalizer. This makes
       it easy to implement resetting threads. Running threads are kept
       in sb-thread::*all-threads*.
    ** threads block all blockable signals when going down:
       interrupt-thread and others cannot catch it at an inappropriate
       moment, for instance calling quit outside the catch %end-of-the-world
    ** target-thread.lisp renamed target-multithread.lisp,
       target-thread.lisp now contains the generic thread support
    ** new file early-thread.lisp: define *current-thread*
    ** removed thread state STOPPING that was only used for assertions and
       complicated matters
    ** renumbered thread states
    ** sb-thread::release-spinlock now releases the locks with
       non-fixnum value, but is no longer safe to call multiple times
    ** much simplified locking for threads and gc
    ** fixed deadlocking bugs introduced by the pthread merge