* elmo-imap4.el (elmo-imap4-session): Added new slot `flags'.

* elmo-imap4.el (elmo-imap4-session): Added new slot `flags'.
(elmo-imap4-session-select-mailbox): Set it according to the SELECT response.
(elmo-folder-open): Ditto.
(elmo-imap4-set-flag): Send store flag command only when the `flags'
slot of the session includes the flag.
(elmo-imap4-session-flag-available-p): New function.
(elmo-imap4-folder-list-flagged): Ditto.
(elmo-folder-list-unreads-plugged): Use it.
(elmo-folder-list-importants-plugged): Ditto.
(elmo-folder-list-answereds-plugged): Ditto.
(elmo-imap4-search-internal-primitive): Ditto.
(elmo-imap4-folder-list-any-plugged,
elmo-imap4-folder-list-digest-plugged): Abolish.