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.

lib-sieve: vnd.dovecot.report extension: Added support for configuring the "From:" address used in the report.

A "vnd.dovecot.config.<identifier>" environment item maps to the content of the matching sieve_env_<identifier> plugin setting.

This was quite fundamentally broken, not only due to previous change.

lib-sieve: editheader extension: Made protection against addition and deletion of headers configurable separately.
Also, the received and auto-submitted headers are now only protected against deletion as required in the RFC.
Updated the documentation accordingly.

Backwards compatibility is provided for vnd.dovecot.duplicate.
Still need to fix the constraint that it must not track duplicates from failed Sieve executions.

doc/extensions/spamtest-virustest.txt: Clarified that these extensions use the extended POSIX regular expressions.

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.