Apply timingsafe_bcmp() in authentication paths

Apply timingsafe_bcmp() in authentication paths

This commit applies timingsafe_bcmp() to authentication paths that
handle attributes or data previously compared with memcpy() or strcmp(),
which are sensitive to timing attacks.

The following data is concerned by this change, some being in the
backend and some in the frontend:
- For a SCRAM or MD5 password, the computed key or the MD5 hash compared
with a password during a plain authentication.
- For a SCRAM exchange, the stored key, the client's final nonce and the
server nonce.
- RADIUS (up to v18), the encrypted password.
- For MD5 authentication, the MD5(MD5()) hash.

Reported-by: Joe Conway <mail@joeconway.com>
Security: CVE-2026-6478
Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Backpatch-through: 14