main, sshd: Refactor authentication, add IP throttling, improve passphrase auth

main, sshd: Refactor authentication, add IP throttling, improve passphrase auth

* Move password authentication handling into sshd/auth (fixes #394).

Password authentication is now completely handeled in Auth. The normal
keyboard-interactive handler checks if passwords are supported and asks
for them, removing the need to override the callbacks.

Brute force throttling is removed; I'd like to base it on IP address
banning, which requires changes to the checks.

I'm not sure, but I think timing attacks against the password are fixed:
- The hashing of the real password happens only at startup.
- The hashing of a provided password is something an attacker can do
themselves; It doesn't leak anything about the real password.
- The hash comparison is constant-time.

* refactor checks, IP-ban incorrect passphrases, renames

- s/assword/assphrase/, typo fixes
- bans are checked separately from public keys
- an incorrect passphrase results in a one-minute IP ban
- whitelists no longer override bans (i.e. you can get banned if you're
whitelisted)

* (hopefully) final changes