Use a prune factor when pruning the registry over the hard limit, instead of 10.

Use a prune factor when pruning the registry over the hard limit, instead of 10.

* gnus-registry.el (gnus-registry-fixup-registry): Set prune-factor to
0.1 expicitly.

* registry.el (prune-factor): New initialization parameter defaulting
to 0.1.
(registry-prune-hard): Use it.