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.

- Now compiles regularly without --with-unfinished-features
- Still experimental though, so be careful.
- Changed command structure a bit, removing the useless -M option.

- Updated documentation.
- Made editheader extension disabled by default

lib-sieve: vacation extension: finally added support for using the original recipient in vacation address check.

Made vnd.dovecot.debug extension available to the LDA plugin instead of only the command line tools.

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

Improved ManageSieve config example; 'sieve' is added to protocols setting in 20-managesieve.conf now.

- No longer track the generated pigeonhole.7
- Some makes don't like the 'handy' notation that was used.
- Deleted trailing spaces
- Escaped some dashes
- Replaced some ' and " by \(aq and \(dq
- Sorted manual pages in section 'See also'
  (first list all man pages from the lowest section (1)
  alphabetical sorted, then the next section …)