* elmo-imap4.el (elmo-imap4-process-folder-list): Use `nconc'

* elmo-imap4.el (elmo-imap4-process-folder-list): Use `nconc'
instead of `append'.
(elmo-imap4-commit): Use `elmo-imap4-plugged-p' instead of
`elmo-plugged-p' directly.
* elmo-pop3.el (elmo-pop3-commit): Use `elmo-pop3-plugged-p' instead
of `elmo-plugged-p' directly.

* elmo-pop3.el (elmo-pop3-get-connection): Split temporary variable.
Eliminate unused variable.  Use `memq' for checking a process status.
Use `cons' instead of `append' for updating the connection cache.
* elmo-nntp.el (elmo-nntp-get-connection): Likewise.
* elmo-imap4.el (elmo-imap4-get-connection): Likewise.