Turn expiry on for nnselect groups

Turn expiry on for nnselect groups

Articles in (persistent) groups from the gnus/nnselect backend will
now be run through the expiry process upon exit, like other persistent
groups. Expiry is not on by default for ephemeral nnselect groups but
may be turned on with nnselect-allow-ephemeral-expiry set to t.

* lisp/gnus/nnselect.el (nnselect-request-expire-articles): Make
article expiry work.
(nnselect-allow-ephemeral-expiry): New variable.
* doc/misc/gnus.texi (Creating Search Groups): Document
nnselect-allow-ephemeral-expiry.