Explicitly define method `elmo-folder-open-internal-p' for POP3 and

Explicitly define method `elmo-folder-open-internal-p' for POP3 and
Shimbun folder class prevent calling this method of the first parent,
elmo-net-folder which falls back to the default behaviour, always
return t.

* elmo-pop3.el (elmo-folder-open-internal-p): Define method for this
class.
* elmo-shimbun.el (elmo-folder-open-internal-p): Dto.
* elmo-multi.el (elmo-folder-open-internal-p): Fix invalid let
statement.