Forward via postfix

Adds an option to mailboxes to forward copy via mta vs. lda. Forwarding in mta (postfix) is more efficient and keeps mail queues cleaner (less bounces filling queues), though forwarding in lda (eg. sieve) acts after spam scanning so will forward less spam (with amavis at least, I don't know if forwarded mail still gets scanned with rspamd).

Fixes #5670 (closed)