Factor out function to send string to session process

* elmo-imap4.el (elmo-imap4-session-process-send-string): New
function. Send string to session process, facoted out from
`elmo-imap4-send-string`.
(elmo-imap4-send-string): Use factord out function.
(elmo-imap4-send-command): Use new function to send to process.