lib-sieve: message body: Fixed assert failure occurring when text extraction is attempted on a empty or broken text part.

Included scripts now execute in the context of the inner parent foreverypart loop.

An empty string yielded an empty string rather than "0".

It now extracts bare text from HTML/XHMTL parts.
Other text/* parts are still returned as is. Any other unrecognized content types are skipped.

This allows setting the MAIL FROM of the SMTP envelope for the notification e-mails.
The syntax is identical to the sieve_redirect_envelope_from setting.

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.

Currently cannot freely assing server metadata items for testing.
Disabled tests for now.

Also added testsuite items to test syntax.

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.

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

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.

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

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.