test suite: enotify extension: Add tests for interaction between ":encodeurl" and variable size limits.

It is a remnant from debugging.

It erroneously applied another layer of MIME header encoding.
The problem is that existing Sieve scripts may rely on this behaviour; i.e. scripts may contain addresesses with literal UTF8 in the phrase part.
Therefore, this change allows both behaviors.

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.

This allows setting the MAIL FROM of the SMTP envelope for the notification e-mails.
The syntax is identical to the sieve_redirect_envelope_from setting.

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

Enotify/Mailto: prevented single recipient from receiving multiple notifications on the same message.