lib-sieve: vacation extension: Compose the "To:" header from full sender...

lib-sieve: vacation extension: Compose the "To:" header from full sender address found in the first "Sender:", "From:" or "Resent-From:" header.

Before, it would create a "To:" header without a phrase part. The new behavior is nicer, since the reply will be addressed to the sender by name if possible.
This addresses an old FIXME.