mail/mlmmj: update to 2.1.0

Changes:

• New $disarmedfrom$ variable for control/customheaders. Expands to the original From: header value with '<', '>', '@' replaced by '(', ')', '=' for safe use in display names ("me@here.com" -> "me=here.com"). Complements $posteraddr$ (raw email address).
• mlmmj-sub: batch subscribe via stdin with -a -. Reads one address per line, skips empty lines and '#' comments.
• MIME type detection and rejection in mlmmj-receive. Prevent multipart/alternative or multipart/mixed messages from being posted to the list (control/mimedeny).
• Fix: fdopendir UB by using dirfd() instead of raw fd
• Various code simplifications (send_digest, bouncemail, do_access, initsmtp)
• Many new unit tests and test fixes
• README: document DKIM/From munging with $posteraddr$ and $disarmedfrom$
• README.sendmail.md: update documentation