The expirable mark of a read article if any is stripped when it
is copied or moved to the group in which to do not automatically
mark read articles as expirable.  That is good.

However, it will not be marked as expirable when copying or moving
a read article (that has not been marked as expirable) to the group
in which to automatically mark read articles as expirable.  So,
such an article will never be expired unless it is read.  Actually
I have a number of such articles in the auto-expirable groups.

The attached patch solves it, i.e., read articles copied or moved
to auto-expirable group are automatically marked as expirable.
Does anyone see any harm with this change?   Though I will not
commit it until the next release anyway.