Clean up codes.

Clean up codes.
(nnshimbun-buffer, nnshimbun-current-directory)
(nnshimbun-current-group, nnshimbun-server-directory): Removed.
(nnshimbun-current-server, nnshimbun-server-directory)
(nnshimbun-current-group, nnshimbun-current-directory): New macros.
(nnshimbun-backlog, nnshimbun-find-parameter): Use
`nnshimbun-current-server'.
(nnshimbun-possibly-change-group, nnshimbun-open-server):
Reimplemented.
(nnshimbun-close-server): Do not kill `nnshimbun-buffer'.
(nnshimbun-request-article-1, nnshimbun-request-list): Use
`erase-buffer' instead of `delete-region'.
(nnshimbun-request-article): Use `nnshimbun-current-group'.
(nnshimbun-request-group): Remove redundant checks.
(nnshimbun-request-scan): Check arguments strictly.
(nnshimbun-retrieve-headers, nnshimbun-retrieve-headers-with-nov):
Reimplemented.
(nnshimbun-generate-nov-database, nnshimbun-search-id)
(nnshimbun-write-nov, nnshimbun-request-expire-articles):
Simplified.
(nnshimbun-nov-buffer-name, nnshimbun-nov-file-name): Make the
first argument optional.
(nnshimbun-open-nov): Bind variables to set coding systems for
path names.