* wl-util.el (toplevel): Require 'elmo-flag'.

* wl-util.el (toplevel): Require 'elmo-flag'.
(wl-read-search-condition): New function (Renamed from
`elmo-read-search-condition').
(wl-read-search-condition-internal): Likewise.

* wl-summary.el (wl-summary-decide-flag): Cause error if flag is
invalid.
(wl-summary-pick): Use `wl-read-search-condition' instead of
`elmo-read-search-condition'.
(wl-summary-virtual): Ditto.

* wl-folder.el (wl-folder-virtual): Ditto.
(wl-folder-pick): Ditto.

* wl-fldmgr.el (wl-fldmgr-make-filter): Ditto.

* elmo-util.el (elmo-read-search-condition): Eliminate (renamed to
`wl-read-search-condition').
(elmo-read-search-condition-internal): Likewise.

* elmo-flag.el (elmo-flag-char-regexp): New constant.
(elmo-flag-valid-p): New function.
(elmo-folder-initialize): Check flag name is valid.