Use empty envelope address where possible - #514

DKIM-sign all email sent from domain (hmailserver#512)
* Sign all emails from local domains.
* Prevent duplicate DKIM signatures
* Use From header instead of envelope from
* Removed redundant validation of  used algorithm

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Fix: Emails stored without trailing newline (hmailserver#513)
The fix for hmailserver#500 introduced an issue where email messages were sometimes missing a trailing newline on disk. This later lead to issues parsing the content.

The code has been updated so that the trailing newline is added, and the parsing has been made more robust.

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Fix: Garbage after closing boundary could lead to truncated data
If an malformed email was processed, data after the malformed portion could be truncated.

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Use empty envelope address where possible (hmailserver#514)

- Fix: IMAP FETCH does not properly honour "<start.size>" clause hmailserver#334 (hmailserver/hmailserver@009e870)
- Fix: Invalid Recent-count in IMAP notifications (hmailserver/hmailserver@2c860c9)
- Removed unnecessary message load (hmailserver/hmailserver@81ecc7f)
- Fold long lines when using QP-encoding, hmailserver#171 (hmailserver#515) (hmailserver/hmailserver@5a5015b)
- Use empty envelope address where possible (hmailserver#514) (hmailserver/hmailserver@5e46b81)
- Fix: Long ZIP filenames and foreign " ö "cause 'block attachment' renaming and conversion errors hmailserver#145 (hmailserver#516) (hmailserver/hmailserver@b73d8fa)

…523)

- Fix some spam scoring inconsistencies, for example the spam-counter was not updated in POP3ClientConnection when a message scored above the SpamDeleteThreshold
- Make sure both the SpamMarkThreshold and SpamDeleteThreshold are > 0
- If SpamMarkThreshold and SpamDeleteThreshold are set to 0, disable this functionality

Add a internal (persistent) FlagSpam message flag as these below are optional and can be disabled
- Add X-hMailServer-Spam
- Add X-hMailServer-Reason
- Add to message subject

Use the internal FlagSpam message flag to (optionally) determine if a message should be forwarded, replied to if defined by either Rule or with a Account Forward/OOF when a message is considered/marked as being spam

webadmin and tests included

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In RuleApplier.cpp the include MailerDaemonAddressDeterminer.h can be removed, there was also a orphaned std::shared_ptr<Account> emptyAccount; that wasn't used
In SMTPVacationMessageCreator.cpp the SetReturnPath action can also be removed, as part of #514