[Security] Add challenge-response auth with scrypt verifiers and stop storing plaintext passwords

Challenge-response authentication: the client derives a scrypt verifier
(RFC 7914, EVP_PBE_scrypt, N=32768, r=8, p=1) and authenticates with
HMAC-SHA256(key, nonce), so neither the password nor its hash is
transmitted. The stored format becomes
"$scrypt$<n>$<r>$<p>$<salt>$<verifier>" and Response_PasswordSalt
now carries the cost parameters. Strict servers only accept scrypt
verifiers; legacy accounts are migrated after a successful login.

Fix #344 for challenge-response servers: a saved profile stores the
derived verifier under the password key instead of the plaintext password.
The connect dialog loads it without revealing it, autoconnect passes it
through, the change-password dialog no longer prefills the old password
field with it, and the client only persists the verifier when
"Save password" is checked.

Took 3 minutes

Took 1 minute

Took 10 seconds

Took 7 minutes

servatrice/servatrice.ini.example

@@ -103,6 +103,16 @@ password=123456
 ; Accept only registered users? default is false (accept unregistered users)
 regonly=false
 
+[security]
+
+; How strictly new authentication features are enforced.  Possible values:
+; * legacy: only accept the legacy 1000-round SHA-512 password hashes;
+; * mixed: accept both legacy hashes and challenge-response authentication (default);
+; * strict: only accept challenge-response authentication from clients that support it,
+;           and reject plain password submissions.  Legacy accounts are migrated to PBKDF2
+;           automatically on their next successful login.
+authentication_strictness=mixed
+
 [users]
 
 ; The minimum length a username can be