Also added testsuite items to test syntax.

This allows extension of test commands that evaluate the message somehow.
Currently this is implemented only for the message header.
Extensions can thereby influence what headers are evaluated and how.

lib-sieve: Implemented sieve_redirect_envelope_from setting, which allows configuring the envelope sender of redirected messages.
Can either be set to the envelope sender, recipient, or orignal recipient of the processed message or it can be set explicitly to a static address.

Based on patch by Timo Sirainen.

Doveadm-sieve plugin may be broken at this point; not tested so far.

NOTE: Dovecot doesn't currently MIME-encode these in the headers.

SMTP API now supports adding multiple recipients to a single SMTP transaction. This makes enotify/notify more efficient, since only one message is actually sent.

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

lib-sieve: spamtest/virustest extensions: Tests were erroneously performed against the original message.
When used together with extprograms filter to add the spam headers, the changes were not being used by the spamtest and virustest extensions.

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

Forgot to rewind mail stream before header parsing.
Wrapped mail stream is obtained long before parsing, so if it is used in the mean time, it must be rewound to the beginning.

Testsuite hostname is now true local hostname.

Some environment items in the base specification are not (yet) applicable to Dovecot and are therefore not supported.

It is still a plugin, but no longer a separate package.

Forgot to increment count for the clone.

It is now possible to track duplicates based on arbitrary headers or even arbitrary string values using the new :header and :value arguments respectively.
The experation time can be configured using the new :seconds argument.
This change is backwards compatible as long as the name argument wasn't used. This is now a :handle <handle> argument.

Added support for the :optional tag of the include command.
Changed Sieve script API in the process; opening the script is now available as a separate step.

Added test for interaction between include extension and dict script location support.

Sieve script equality function implementation was wrong.

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

  - Built generic interface for alternative script sources.
  - Implemented dict script location.
  NOTE: ManageSieve will not work with this yet, nor will sieve_before/sieve_after.

This puts the ".sieve" and ".svbin" file extension definitions in a single locatio.
IMPORTANT: this fixes a bug in the include extension that implicitly mapped script names like "name.sieve" to "name".

This was broken by previous change. It now properly uses the final version of the message
instead of the original.