lib-storage: Add struct mailbox_status.flags

This is similar to permanent_flags, except it tells which flags can be
changed at all in the session. Mainly it allows plugins to make changes to
IMAP's untagged FLAGS reply.

src/lib-storage/index/index-status.c

@@ -185,6 +185,7 @@ void index_storage_get_open_status(struct mailbox *box,
 			status_r->allow_new_keywords =
 				!box->disallow_new_keywords;
 		}
+		status_r->flags = MAIL_FLAGS_NONRECENT;
 	}
 }
 

src/lib-storage/mail-storage.h

@@ -258,6 +258,8 @@ struct mailbox_status {
 
 	/* These flags can be permanently modified (STATUS_PERMANENT_FLAGS) */
 	enum mail_flags permanent_flags;
+	/* These flags can be modified (STATUS_PERMANENT_FLAGS) */
+	enum mail_flags flags;
 
 	/* All keywords can be permanently modified (STATUS_PERMANENT_FLAGS) */
 	bool permanent_keywords:1;