[LBBS-31] mod_sieve: Major Sieve filtering overhaul and improvements

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Referencing Issues

[LBBS-54] mod_mailscript: Add RECIPIENT condition

[LBBS-107] mod_sieve: Segfault in lib libsieve_strbuffree during message delivery

Feature capabilities

• Check for nonexistent folders during Sieve script validation and warn if the referenced mailbox does not exist.
• Ideally, cache the filtering script in memory and don't reparse each invocation by the SMTP server
• Need to be able to move messages to remote IMAP folders (in .imapremote). Challenge here is we can't use the imap_client subsystem of the BBS (or at least we don't now, maybe we should)
• Client-side IMAP filtering, e.g. this kind of thing: https://github.com/lefcha/imapfilter - but fully transparent, using normal Sieve filtering script syntax, just a separate script for post-delivery processing
• .forward should be honored: https://www.man7.org/linux/man-pages/man5/forward.5.html

Pathway:

• Likely need my own library, since libsieve is not being maintained and won't build (will need to be compiled from source to fix bugs and add features). See https://github.com/sodabrew/libsieve/issues/4
• Improve Sieve support
  • For example, Sieve extension for external programs: https://github.com/dovecot/pigeonhole/blob/main/doc/rfc/spec-bosch-sieve-extprograms.txt
  • All Sieve extensions: https://www.iana.org/assignments/sieve-extensions/sieve-extensions.xhtml
  • Also see: https://support.tigertech.net/sieve
• MailScript: Since mproc->recipient is NULL for outgoing messages (in both mod_mailscript and mod_sieve), and is also not set in system rules (outside of per-user) rules processing, add a RECIPIENTS condition, which can be used for outgoing messages. Another niggly point - some things make more sense to do in a user rule, but if that requires network access, it can't currently run in the container
• MailScript/Sieve: Be able to have rules based on the user part of an email address only (match without the subaddress) - for example, page-me+topic1@example.com, page-me+topic2@example.com, and filter based on just the page-me part if needed
• MailScript: If a script error occurs, have the ability to email that to the user (though we don't want to generate recursive error messages, so this message should bypass filtering?...)

Links: https://github.com/simta/simvacation/blob/main/simvacation.c

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