DEFCON 4 and lower were blocking SAREGISTER. This is wrong; admins should be
allowed to make new accounts even under DEFCON (this may be needed
specifically to work around the DEFCON restriction).

NS SAREGISTER would fail due to a nil dereference of `client`;
add two safeguards against this.

Account registrations pending verification should produce a logline and
send a sno.

Reported by @tacerus. This was most likely introduced by 8b2f6de3e.

See #1676

* 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.

reported by @Mikaela

See #1659

Users logged into an account can see their own email.
Opers with the accreg capability can see all users.

See #1603

Track channel join times, use them to optionally enforce history access
restrictions

enable the use of grouped nicks as account names for SASL,
if force-nick-equals-account is disabled

Store the channel-user modes of always-on clients along with their
channel memberships, restore them on server startup. This will coexist
alongside /CS AMODE, which autoapplies modes to clients on join regardless
of their always-on status.

NS SUSPEND DEL incorrectly required the use of the casefolded account name.

Fixes #1380

Enhancements to NS SUSPEND, including stored metadata and the ability
to list suspensions

0. Enable email verification
1. Register `dog`
2. Register `d0g`
3. Verify `dog`
4. Verify `d0g`: verification succeeds but the nick cannot be used

See discussion on #68.