* elmo-vars.el (elmo-imap4-stream-type-alist): New variable.

* elmo-vars.el (elmo-imap4-stream-type-alist): New variable.
(elmo-pop3-stream-type-alist): New variable.
(elmo-nntp-stream-type-alist): New variable.

* elmo-util.el (elmo-buffer-exists-p): Rewrite as function.
(elmo-kill-buffer): Ditto.
(elmo-get-network-stream-type): Add 2nd argument `stream-type-alist'.
(elmo-network-get-spec): Add 5th argument `stream-type-alist'.
(elmo-nntp-get-spec): Take into account `elmo-nntp-stream-type-alist'.
(elmo-pop3-get-spec): Take into account `elmo-pop3-stream-type-alist'.
(elmo-imap4-get-spec): Take into account
`elmo-imap4-stream-type-alist'.