This adds the vnd.dovecot.default-mailbox and vnd.dovecot.username environment items.
It creates the 'env' variables namespace through which all environment items can be accessed directly.
This extension is yet to be documented properly.
More environment items are under consideration.

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

This is still an unfinished feature.

Also contains a few other small fixes by Timo.

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

- Added sieve_global_extensions setting.

- Updated documentation.
- Made editheader extension disabled by default

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

Fixed segfault bug in extension configuration, triggered when unknown extension is mentioned in sieve_extensions setting.

Fixed default Sieve capability: extra extensions spamtest, spamtestplus and virustest were also enabled by default.