lib-sieve: updated rfc2822 header field body verification to exclude non-printing characters (RFC5322).

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.

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.