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

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

Message headers produced from user-supplied data are now RFC2047-encoded if necessary for outgoing messages.

Testsuite: added tests on the envelope of outgoing messages produced by redirect, enotify:mailto, reject and vacation.