fixes #921: do not change seen/unseen flag by connecting to imap with /readonly flag

when UNCHANGED is set in mailbox settings of mailconverter the seen/unseen status of any mail shall stay as it is, after the mail was scanned. To achive this, the imap account will be connected in readonly mode (/readonly flag set). with this trick the /seen flag of any email will not be touched.

mentioned in issue #921 (closed)

Status changed to merged

mentioned in commit f4b031dd