control: action to override a single fan in a zone - Lock fan in Zone

The override Action can only reference fans by name. Thus, Zone is
modified to apply a target lock to a specific fan given by name.
Additionally, the unlock method must pass the target value to allow
support for concurrent locks.

Signed-off-by: Mike Capps <mikepcapps@gmail.com>
Change-Id: Ic2ea81f318fb2c866198a4b24e38cc679147afc5
diff --git a/control/json/fan.hpp b/control/json/fan.hpp
index 2a10846..116e50f 100644
--- a/control/json/fan.hpp
+++ b/control/json/fan.hpp
@@ -42,6 +42,8 @@
  */
 class Fan : public ConfigBase
 {
+    friend class Zone;
+
   public:
     /* JSON file name for fans */
     static constexpr auto confFileName = "fans.json";