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.

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

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

Additional paths are specified by adding a sequence number (starting at 2) to the setting name.
Added support for specifying home-relative paths for sieve_before and sieve_after.
This enables administrators to specify user-specific global Sieve scripts that cannot be viewed or changed by virtual users.

  - 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.

Old name is now deprecated.
Support for the even older global_script_path name for this setting is dropped.

Setting sieve_max_redirects=0 now means that redirects are prohibited rather than unlimited.
Now there is always a limit, but it can be enormous if the administrator wants to.

- Added sieve_global_extensions setting.

- Updated documentation.
- Made editheader extension disabled by default

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