Allow SSH keys on multiple accounts

Update validation and duplicate checks to scope key uniqueness per
account, and clarify the console error message. Document cross-account
key assignment and authentication testing.

src/user_console.c

@@ -357,7 +357,7 @@ static int add_key(const char *username)
     secure_wipe(line, sizeof(line));
     if (error != ESP_OK) {
         if (error == USER_DATABASE_ERR_DUPLICATE_SSH_KEY) {
-            printf("Could not add SSH key: that public key is already assigned to an account.\n");
+            printf("Could not add SSH key: that public key is already assigned to this 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);