+/**
+ * Check existence and permissions of a key file.
+ *
+ * \param file The path of the key file.
+ * \param private_key Whether this is a private key.
+ *
+ * This checks whether the file exists. If it is a private key, we additionally
+ * check that the permissions are restrictive enough. It is considered an error
+ * if we own the file and it is readable for others.
+ *
+ * \return Standard.
+ */