Allow specifying TCP4 or TCP6 for outgoing email sending, or choosing a
specific local address to send from.

Send Message-ID even if DKIM is not enabled, for compatibility with Gmail:

* A workaround for Ergo 2.12.0 is to enable DKIM
* You need to enable either DKIM or SPF (preferably both) to send to Gmail anyway
* You also need forward-confirmed reverse DNS, which can be tricky for IPv6...

* Fix #1997 (allow the use of an external file for the email blacklist)
* Change config key names for blacklist (compatibility break)
* Accept globs rather than regexes for blacklist by default
* Blacklist comparison is now case-insensitive

Fixes #2048

* Add email-based password reset

Fixes #734

* rename SETPASS to RESETPASS

* review fixes

* abuse mitigations

* SENDPASS and RESETPASS should both touch the client login throttle
* Produce a logline and a sno on SENDPASS (since it actually sends an email)

* don't re-retrieve the settings value

* add email confirmation for NS SET EMAIL

* smtp: if require-tls is disabled, don't validate server cert

* review fixes

* remove cooldown for NS SET EMAIL

If you accidentally set the wrong address, the cooldown would prevent you
from fixing your mistake. Since we touch the registration throttle anyway,
this shouldn't present more of an abuse concern than registration itself.

See #1659

Fixes #1510