Adding 'instanceExists()' static methods to THREE of the RplSetMgr subclasses.

This is needed because an enhancement to RplSet::populateNameSpace()
for the development of Global RPL Functions causes infinite recursion via
RplSetMgr::allOpenSets().  This method is not the place to be instantiating
RplSet Managers, and it doesn't need to. 

-----------------
Opt/OptSetMgr.cpp
Opt/OptSetMgr.hpp
-----------------

   static bool instanceExists();

   bool OptSetMgr::instanceExists()
   {
      return (_optSetMgr != NULL);
   }

------------------
Rpl/RuleSetMgr.cpp
Rpl/RuleSetMgr.hpp
------------------

   static bool instanceExists();

   bool RuleSetMgr::instanceExists()
   {
      return (_ruleSetMgr != NULL);
   }

----------------------
Sim/RplExprSlotMgr.hpp
----------------------

   inline static bool instanceExists();

   inline bool RplExprSlotMgr::instanceExists()
   {
      return (instance != NULL);
   }

===================================

-----------------
Rpl/RplSetMgr.hpp
Rpl/RplSetMgr.hpp
-----------------

(1) New method:

   // Get the groups in all currently open Global RPL sets, 
   // optionally excluding the specified 'exclude' RplSet.
   void getGlobalGroups (cwSlist<RplGroup*>& retList,
                         RplSet* excludeRplSet = NULL) const;

(2) void RplSetMgr::allOpenSets(Rpl::SetList& sets)

   Process the various RplSetMgrs only if they exist. Specifically, don't
   force them to be instantiated -- if they don't yet exist, then they 
   certainly don't contain any open RplSets.

   The unnecessary instantiation, in the context of this method, was causing
   infinite recursion, with the use of the new getGlobalGroups method in
   RplSet::populateNameSpace().

---