Log Message:
mail/dovecot2-pigeonhole: Update to 0.5.21

Changelog:
- sieve: Using the deleteheader action on a message with a broken/invalid
  header can cause the Sieve interpreter to crash with an assert panic.
  This can happen e.g. when the message is missing the empty EOH line
  between the headers and the body of the message. Fixes:
  Panic: file edit-mail.c: line 820 (edit_mail_headers_parse):
  assertion failed: (body_offset > 0).
- sieve: Pigeonhole added an extra Message-ID header during mail
  forwarding when the existing one was invalid. Now it adds the
  Message-ID only if it is entirely missing. Existing Message-ID(s) are
  left unchanged.