lib-sieve: redirect action: Always set X-Sieve-Redirected-From header to sieve_user_email if configured.

Set it to the final recipient only when there is an envelope and sieve_user_email is unconfigured.

This is used mainly when no other means of obtaining the user's email is available, such as when there is no envelope (in IMAP).
Before, the postmaster address or the null "<>" address was used as a fallback for sending messages.
The redirect and enotify commands can be explicitly configured to use what is configured with sieve_user_email.

lib-sieve: Made sure local part of a mail address is encoded properly using quoted string when it is not a dot-atom.

lib-sieve: Implemented sieve_redirect_envelope_from setting, which allows configuring the envelope sender of redirected messages.
Can either be set to the envelope sender, recipient, or orignal recipient of the processed message or it can be set explicitly to a static address.

Testsuite: added support for testing delivered messages by looping these back as the evaluated message. Added tests for the fileinto command.

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