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.

Fixed various compile warnings related to spurious semicolons and inappropriate variable initialization.

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