1.0.6.7: thread-safe UPDATE-DFUN

1.0.6.7: thread-safe UPDATE-DFUN

 * Make GET-SPINLOCK detect unwanted recursion. Despite the old
   comments in GET/RELEASE-SPINLOCK, we can store EQ-comperable lisp
   objects in SPINLOCK-VALUE -- just like we do for mutexes.
   (Potentially freshly consed bignums that the old comments referred
   to are not sanely EQ-comperable, of course.)

 * Implement WITH-RECURSIVE-SPINLOCK.

 * Adjust thread.impure.lisp accordingly.

 * Add a per generic function spinlock. (We could use mutexes, but
   since contention is presumed to be rare we don't want to pay the
   wakeup syscall cost for every UPDATE-DFUN call: if and when our
   mutexes get smart doing the wakeup only when there are threads
   waiting we can and should switch this -- and probably almost all
   uses of spinlocks -- to mutexes.) This spinlock is grabbed to
   ensure that the dfun state, fin function, and name are all updated
   atomically.