* nnmaildir.el: Use defstruct. Use a single copy of

* nnmaildir.el: Use defstruct.  Use a single copy of
nnmail-extra-headers to save memory.  Store server's group name
prefix instead of each group's prefixed name.
* nnnil.el (nnnil-retrieve-headers, nnnil-request-list): Erase
nntp-server-buffer.