Clean up nnimap buffers with dead processes

Clean up nnimap buffers with dead processes

* lisp/gnus/nnimap.el (nnimap-keepalive): If the keepalive "NOOP"
fails, remove the buffer with the dead process from
`nnimap-process-buffers' and `nnimap-connection-alist'.
(nnimap-find-connection): Do the same here, when the connection can't
be found.
(nnimap-close-server): Remove process buffer from lists when closing
server.