[Dovecot] INBOX of a shared namespace appears to be always subscribed

[Holger Richter]

I've repeated the tests with LSUB "" *. I think there's a problem with 
setting the namespace prefix.

telnet imap.xx.yy 143
	1 LOGIN user password
	2 LSUB "" *            => shows INBOX
	                                Sent
	                                Trash
	                                folderAA
	                                shared/UserB/folderXX
	3 LSUB "" *            => shows shared/UserB/INBOX
	                                shared/UserB/folderXX
	                                Sent
	                                Trash
	                                folderAA
	4 LSUB "" *            => shows shared/UserB/INBOX
	                                shared/UserB/folderXX
	                                Sent
	                                Trash
	                                folderAA

This behavior is reproducable. Please note that the mailboxes of the 
shared namespace are listed first and the own inbox of the user disappears.

Holger