nnmaildir.el: Silence lexical warnings

nnmaildir.el: Silence lexical warnings

* nnmaildir.el (nnmaildir--prepare): Use a more functional style.
(nnmaildir--update-nov): Remove unused var `numdir'.
(nnmaildir-request-type, nnmaildir--scan, nnmaildir-request-newgroups)
(nnmaildir-request-group, nnmaildir-request-create-group)
(nnmaildir-request-post, nnmaildir-request-move-article)
(nnmaildir-request-accept-article, nnmaildir-active-number):
Mark unused args.
(nnmaildir-get-new-mail, nnmaildir-group-alist, nnmaildir-active-file):
Declare.
(nnmaildir-request-scan): Remove unused vars `group' and `grp-dir'.
(nnmaildir-request-update-info): Remove unused vars `dotfile', `num',
`mark', `end', `new-mark', and `mark-sym'.
(nnmaildir-retrieve-headers):
Remove unused args `srv-dir', `dir', `nlist2'.
(nnmaildir-request-expire-articles):
Remove unused vars `article', `stop' and `nlist2'.
(nnmaildir-request-set-mark): Remove unused vars `begin', `article' and
`end'.  Use nnmaildir--article when dyn-binding is needed.
Give the value directly in the `let' for `del-mark', `del-action',
`add-action', and `set-action'.  Don't use `add-to-list' on a local var.
(nnmaildir-close-server):
Declare those local vars that need to be dyn-bound.