lib-sieve: Changed sieve_extension_override() to use hash_table_update() rather than hash_table_insert().

Due to a Dovecot change this would now cause an assertion failure.

This would uselessly allocate a new entry with the same content.

These are not available under that name anymore and when the replacing extension has the same bare name, this would have led to duplicate capabilities.

It allows sending feedback report (RFC 5965) messages.

The extensions listed in this setting do not need to be enabled explicitly using the Sieve require command.
This behavior voilates the standard, but is necessary for compatibility with some existing implementations of Sieve.
Do not use this setting unless you really need to!

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.