* gnus-cache.el (gnus-cache-save-buffers): Use
gnus-cache-update-overview-total-fetched-for to change disk space
used by this group.
(gnus-cache-possibly-enter-article): Use
gnus-cache-update-file-total-fetched-for to increment disk space
used by this group.
(gnus-cache-possibly-remove-article): Use
gnus-cache-update-file-total-fetched-for to decrement disk space
used by this group.
(gnus-cache-generate-nov-databases): Purge total fetched cache.
(gnus-cache-rename-group): New function.
(gnus-cache-delete-group): New function.
(gnus-cache-inhibit-update-total-fetched-for): New variable.
(gnus-cache-need-update-total-fetched-for): New variable.
(gnus-cache-with-refreshed-group): New macro.
(gnus-cache-update-file-total-fetched-for): New function.
(gnus-cache-update-overview-total-fetched-for): New function.
(gnus-cache-rename-group-total-fetched-for): New function.
(gnus-cache-delete-group-total-fetched-for): New function.
(gnus-cache-total-fetched-for): New function.