Use list to build final command string

* elmo-imap4.el (elmo-create-folder-plugged): Remove leading and
trainling space from IMAP command particle.
(elmo-imap4-session-select-mailbox)
(elmo-imap4-session-unselect-mailbox, elmo-imap4-clear-login)
(elmo-imap4-server-diff-async, elmo-imap4-folder-status-plugged)
(elmo-folder-list-subfolders, elmo-folder-delete)
(elmo-folder-rename-internal, elmo-imap4-copy-messages)
(elmo-folder-delete-messages-plugged)
(elmo-imap4-folder-diff-plugged, elmo-folder-open)
(elmo-folder-create-plugged, elmo-folder-append-buffer)
(elmo-folder-next-message-number-plugged)
(elmo-message-fetch-field): Dto.
(elmo-imap4-send-command): Use a list to build final command string.

Building a list makes it easier to ensure that there is only one SPC
between the command particles.