This change matches identical changes for Dovecot's imap-login and pop3-login.

lib-sieve: vacation: handled FIXME regarding the use of variables in the :handle argument. Variables are now handled correctly.

lda-sieve: fall back to global recipient_delimiter setting if plugin/recipient_delimiter is not set.

Configuration is now only read once at extension initialization.

variables extension: fixed deinitialization problem (unfreed variable scope data) newly found by Valgrind.

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.