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.

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

doc/extensions/spamtest-virustest.txt: Clarified that these extensions use the extended POSIX regular expressions.

Use sieve_vacation_send_from_recipient to enable this behavior.
Documentation in doc/extensions/vacation.txt is updated.

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