Added support for the :optional tag of the include command.
Changed Sieve script API in the process; opening the script is now available as a separate step.

Added test for interaction between include extension and dict script location support.

Sieve script equality function implementation was wrong.

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

  - 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.

This puts the ".sieve" and ".svbin" file extension definitions in a single locatio.
IMPORTANT: this fixes a bug in the include extension that implicitly mapped script names like "name.sieve" to "name".

This was broken by previous change. It now properly uses the final version of the message
instead of the original.

It did not use the original message in this case.

Configuration is now only read once at extension initialization.

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

lib-sieve: optimized compilation of tests that yield constant results (i.e. known at compile tme), such as true and false.
If the result of a test is known at compile time, it is optimized away. If an if-command depends on an entirely constant test, it is optimized away as well,
causing only the 'true' sub-block to be compiled.

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

Imap4flags: fixed bug in setflag command; when parameter was a stringlist, only the last item was actually set.