TLS limitations

[Roland McGrath]

> that the counter can "only" be increment 2^32 times before it leads to
> problems.  And there will be problems if this should happen.

Well, put in an assert that crashes when it wraps to zero.  If that ever
happens, we can implement some ugly thing that stops the world and runs
through all the threads eagerly updating them at the wraparound, or
something like that.  But having it definitely crash should it ever happen
is the best way to convince anyone that it really never happens in reality.