[libcalamares] Add Redacted logging

- this was an internal class for logging commands, let's lift
  it up to the Logger framework where it might be more generally
  useful (or not .. everything needs special-casing for actual
  redaction).

src/libcalamares/utils/Logger.cpp

@@ -228,4 +228,28 @@ toString( const QVariant& v )
     }
 }
 
+QDebug&
+operator<<( QDebug& s, const Redacted& l )
+{
+    // Special case logging: don't log the (encrypted) password.
+    if ( l.list.contains( "usermod" ) )
+    {
+        for ( const auto& item : l.list )
+            if ( item.startsWith( "$6$" ) )
+            {
+                s << "<password>";
+            }
+            else
+            {
+                s << item;
+            }
+    }
+    else
+    {
+        s << l.list;
+    }
+
+    return s;
+}
+
 }  // namespace Logger

src/libcalamares/utils/Logger.h

@@ -207,6 +207,25 @@ public:
     const QVariantMap& map;
 };
 
+/** @brief When logging commands, don't log everything.
+ *
+ * The command-line arguments to some commands may contain the
+ * encrypted password set by the user. Don't log that password,
+ * since the log may get posted to bug reports, or stored in
+ * the target system.
+ */
+struct Redacted
+{
+    Redacted( const QStringList& l )
+        : list( l )
+    {
+    }
+
+    const QStringList& list;
+};
+
+QDebug& operator<<( QDebug& s, const Redacted& l );
+
 /**
  * @brief Formatted logging of a pointer
  *