Replace MgmKey::set with MgmKey::{set_default, set_manual} (#224)

* Add MgmKey::set_default method

This wipes any metadata related to derived and PIN-protected management
keys, returning the management key to its default state.

* Transaction::set_mgm_key: Take touch requirement as bool

The Option<u8> was inherited from the original C code's usage of an
unsigned char. We don't need that flexibility, because only two cases
are supported.

* Replace MgmKey::set with MgmKey::set_manual

MgmKey::set_default is now implemented as a wrapper around
MgmKey::set_manual, as they both require clearing metadata related to
derived and PIN-protected management keys.

src/transaction.rs

@@ -229,14 +229,8 @@ impl<'tx> Transaction<'tx> {
 
     /// Set the management key (MGM).
     #[cfg(feature = "untested")]
-    pub fn set_mgm_key(&self, new_key: &MgmKey, touch: Option<u8>) -> Result<(), Error> {
-        let p2 = match touch.unwrap_or_default() {
-            0 => 0xff,
-            1 => 0xfe,
-            _ => {
-                return Err(Error::GenericError);
-            }
-        };
+    pub fn set_mgm_key(&self, new_key: &MgmKey, require_touch: bool) -> Result<(), Error> {
+        let p2 = if require_touch { 0xfe } else { 0xff };
 
         let mut data = [0u8; DES_LEN_3DES + 3];
         data[0] = ALGO_3DES;