Patch by Andreas Schulze.

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.

When this is enabled for a mailbox rule, the specified Sieve script is executed for the message in the source mailbox during a "COPY" event.
This happens only after the Sieve script that is executed for the corresponding message in the destination mailbox finishes running successfully.

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

LDA Sieve plugin: Implemented support for a "discard script" that is run when the message is going to be discarded.

This adds a new "sieve_discard" setting that points to the location of the "discard script".
Once the normal scripts finish executing and the result is that the message will be discarded without being delivered anywhere, the "discard script" is executed.
The discard script can prevent dropping the message, by executing alternative actions.
If the discard script does nothing, the message is still discarded.

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

extprograms plugin: Clarified the language specification regarding handling of filter and execute errors.

This is used mainly when no other means of obtaining the user's email is available, such as when there is no envelope (in IMAP).
Before, the postmaster address or the null "<>" address was used as a fallback for sending messages.
The redirect and enotify commands can be explicitly configured to use what is configured with sieve_user_email.

doc/example-config: Added description for a few generic settings that were only mentioned in the INSTALL file.

It allows sending feedback report (RFC 5965) messages.

Sieve extprograms plugin: Made line endings configurable for the input passed to the external program.

Forgot to adjust a few things when the location types were introduced and documented.

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.

This allows performing ManageSieve actions at command line or through doveadm server.

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