Add option to validate pointer addresses in variable server clients

Added a flag called validate_address to each variable server thread.  When
activated each pointer address will be tested to see if it is in memory
the memory manager is tracking.  If it is then everything proceeds normally.
If it does not, then an error return value is returned for the value of this
variable.

refs #193

trick_source/sim_services/VariableServer/VariableServerThread.cpp

@@ -24,11 +24,11 @@ Trick::VariableServerThread::VariableServerThread(TCDevice * in_listen_dev) :
     binary_data = false;
     multicast = false;
     byteswap = false ;
-    retry_bad_ref = false ;
 
     pause_cmd = false ;
     exit_cmd = false ;
 
+    validate_address = true ;
     send_stdio = false ;
 
     update_rate = 0.1 ;