Threads should not be Cloneable

[Florian Weimer]

* David Schlosnagle:

> I may be missing something, but if java.lang.Thread.clone() will now
> throw CloneNotSupportedException, how would subtypes properly
> implement Cloneable? If the subtype calls super.clone(),
> CloneNotSupportedException will be thrown. If the subtype calls its
> own constructor, that would seem to violate the clone contract for any
> further subtypes.

The clone contract is extremely lose, so this isn't an issue.