OpenSSL uses PKCS#5 padding, and the decryption code was not removing it
correctly. In some cases, this causes the last line of the decrypted config to
be mangled and produces invalid results.
To support this:
- Move config loading to gauth.LoadConfigFile.
- Inject a hook to read the user's password.
- Add unit tests that decryption doesn't corrupt the result.
- Update module dependencies.
- Update Go versions in CI, and fix some config-check warnings.
