Cache web authentication results

Add a short-lived, HMAC-keyed cache for validated principals and
invalidate entries when principals become stale. Improve duplicate SSH
key
errors and enable Ed25519 streaming verification.

src/user_console.c

@@ -356,7 +356,14 @@ static int add_key(const char *username)
     secure_wipe(blob, sizeof(blob));
     secure_wipe(line, sizeof(line));
     if (error != ESP_OK) {
-        printf("Could not add SSH key: %s\n", esp_err_to_name(error));
+        if (error == USER_DATABASE_ERR_DUPLICATE_SSH_KEY) {
+            printf("Could not add SSH key: that public key is already assigned to an account.\n");
+        } else if (error == ESP_ERR_NO_MEM) {
+            printf("Could not add SSH key: the account already has %u keys.\n",
+                   USER_DATABASE_MAX_SSH_KEYS_PER_USER);
+        } else {
+            printf("Could not add SSH key: %s\n", esp_err_to_name(error));
+        }
         return 1;
     }
     revoke_user_network_sessions(username);