mail/dovecot-pigeonhole: update to 0.4.21.

Changelog v0.4.21:

• redirect action: Always set the X-Sieve-Redirected-From header to sieve_user_email if configured. Before, it would use the envelope recipient instead if available, which makes no sense if the primary e-mail address is available.

+ vacation extension: Allow ignoring the envelope sender while composing

the "To:" header for the reply. Normally, the "To:" header is composed
from the address found in the "Sender", "Resent-From" or "From"
headers that is equal to the envelope sender. If none is then found,
the bare envelope sender is used. This change adds a new setting
"sieve_vacation_to_header_ignore_envelope". With this setting enabled,
the "To:" header is always composed from those headers in the source
message. The new setting thus allows ignoring the envelope, which is
useful e.g. when SRS is used.

+ vacation extension: Compose the "To:" header from the full sender

address found in the first "Sender:", "From:" or "Resent-From:"
header. Before, it would create a "To:" header without a phrase part.
The new behavior is nicer, since the reply will be addressed to the
sender by name if possible.

• LDA Sieve plugin: Fixed sequential execution of LDAP-based scripts. A missing LDAP-based script could cause the script sequence to exit earlier.
• sieve-filter: Removed the (now) duplicate utf8 to mutf7 mailbox name conversion. This caused problems with mailbox names containing UTF-8 characters. The Dovecot API was changed years ago, but apparently sieve-filter was never updated.