Log Message:
Update to 1.4.3. From the changelog:

- limit maildir nesting depth
- enable embedding arbitrarily long strings into IMAP commands
- CVE-2021-3578: fix handling of unexpected APPENDUID response code
- don't crash on malformed CAPABILITY responses
- tolerate INBOX mis-casing in Path
- make UIDVALIDITY recovery more strict about vanished messages
- improve error messages about irrecoverably changed UIDVALIDITY
- CVE-2021-20247: reject funny mailbox names from IMAP LIST/LSUB
- be more tolerant of formally malformed response codes
- fix bogus continuation of IMAP list parsing
- accept unsolicited FETCH responses (without payload) after all
- use correct <poll.h> header