proper permission for secrets aka 0o600

When creating secret keys or use the out file feature, the material
shouldn't be readble to everyone by default.

Fix: #260

Signed-off-by: Paul Spooren <mail@aparcar.org>

secret-memory/src/file.rs

@@ -4,4 +4,5 @@ pub trait StoreSecret {
     type Error;
 
     fn store_secret<P: AsRef<Path>>(&self, path: P) -> Result<(), Self::Error>;
+    fn store<P: AsRef<Path>>(&self, path: P) -> Result<(), Self::Error>;
 }