lib-sieve: vacation extension: Allow ignoring the envelope sender while composing the "To:" header for the reply.

This adds a new setting "sieve_vacation_to_header_ignore_envelope".
With this setting enabled, the "To:" header is always composed from the first "Sender", "Resent-From" or "From" header found in the message (in that order).
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.
The new setting allows ignoring the envelope, which is useful e.g. when SRS is used.

Use sieve_vacation_send_from_recipient to enable this behavior.
Documentation in doc/extensions/vacation.txt is updated.

Incorporated sieve_duplicate plugin into main Pigeonhole tree as a normal extension (vnd.dovecot.duplicate).
Also restructured doc dir a bit in the process.

lib-sieve: vacation extension: finally added support for using the original recipient in vacation address check.

Vacation extension: added min/max period configuration settings and changed the internal unit from days to seconds.