Add CLI interface for the signing key

- 'signkey set' sets a new signing key (disables an old one if
  existing).
- 'signkey show' prints the fingerprint.
- 'signkey dump' prints the certificate of the signing key.

src/types/introducer.rs

@@ -83,8 +83,7 @@ impl Introducer {
             cert,
             crypto::Role::Introducer,
             true,
-            Some(domains),
-            None)
+            Some(domains))
     }
 
     /// Remove an introducer.
@@ -102,7 +101,6 @@ impl Introducer {
             cert,
             crypto::Role::Introducer,
             false,
-            None,
             None)
     }
 }