chore: Reorganize memfd secret policy

- Policy is now set in main.rs, not cli.rs.
- Feature is called experiment_memfd_secret, not enable_memfd_alloc

This also fixes the last remaining warnings.

rosenpass/Cargo.toml

@@ -53,5 +53,5 @@ procspawn = {workspace = true}
 
 [features]
 enable_broker_api = ["rosenpass-wireguard-broker/enable_broker_api"]
-enable_memfd_alloc = []
+experiment_memfd_secret = []
 experiment_libcrux = ["rosenpass-ciphers/experiment_libcrux"]