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.

Vacation extension: from address of reply is now by default equal to whatever known recipient alias matched the headers of the message.

Spamtest and virustest extensions: discarded whitespace at beginning of regexp and added syntax testsuite tests.

Fixed small bug in the i;ascii-numeric comparator and added extensive relational tests to the testsuite.