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

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.