Bug report: IMAP APPENDLIMIT behave chaos

Reproduce the issue using the Docker configuration, the maddy imap-acct appendlimit command, and an IMAP client calling STATUS on INBOX. Start at the IMAP STATUS and APPENDLIMIT handling, compare the account and mailbox limits shown by the SQLite query, and verify that an unlimited mailbox reports APPENDLIMIT NIL rather than 0.

Describe the bug

IMAP APPENDLIMIT settings behave in effected, conflict with maddy settings.

Steps to reproduce

1. Run maddy in Docker as instructed in https://maddy.email/docker/
2. Create a creds and imap-acct for an account
3. Now we can expect this account has no LIMIT (NIL) with APPENDLIMIT because account and mboxes are both has no limit.

# docker exec -it maddy maddy imap-acct appendlimit foxcpp@maddy.test
No limit

# sqlite3 imapsql.db 'select * from mboxes where msgsizelimit is null'
1|1|INBOX|1|0||7|1439825317||5
2|1|Sent|1|0||1|3335251900|\Sent|0
3|1|Trash|1|0||1|3434545230|\Trash|0
4|1|Junk|1|0||1|418662061|\Junk|0
5|1|Drafts|1|0||1|1872697851|\Drafts|0
6|1|Archive|1|0||2|556772642|\Archive|0

4. But in effect, this account has an APPENDLIMIT to ZERO

# python
>>> import imaplib
>>> s=imaplib.IMAP4_SSL('127.0.0.1')
>>> s.login('foxcpp@maddy.test', 'foxcpp')
('OK', [b'[CAPABILITY IMAP4rev1 LITERAL+ SASL-IR CHILDREN UNSELECT MOVE IDLE APPENDLIMIT I18NLEVEL=1 SORT THREAD=ORDEREDSUBJECT COMPRESS NAMESPACE] LOGIN completed'])
>>> s.status('INBOX', '(APPENDLIMIT)')
('OK', [b'INBOX (APPENDLIMIT 0)'])

According to RFC7889 The IMAP APPENDLIMIT Extension, the APPENDLIMIT flag in s.login() indicates server support this extension and client will need to discover upload limits for each mailbox. And the STATUS Response indicates server will not accept any APPEND commands at all for the affected mailboxes.

I am expecting to see the following STATUS response, which indicates the mailbox has no limit.

>>> s.status('INBOX', '(APPENDLIMIT)')
('OK', [b'INBOX (APPENDLIMIT NIL)'])

Log files

No need

Configuration file

Exactly same as https://github.com/foxcpp/maddy/blob/fa47d40f6d510a431d2bbc238c7d36a58774ae2f/maddy.conf.docker

Environment information

• maddy version: 0.8.1