Description:

The new `sieve_mdn_original_recipient_header` allows an install to get the
original recipient from a header other than Original-Recipient.


Config changes:

This adds `sieve_mdn_original_recipient_header`, a string option which, if set,
will cause Cyrus to look for the original recipient of Sieve-rejected messages
somewhere other than the `Original-Recipient` field of the messages.


Upgrade instructions:

No action required.


GitHub issue:

None.
