CONTRACTS: use vars auxiliary for DFCC

The `is_auxiliary` flag is now set for instrumentation variables
created for DFCC. This will reduce noise in counter examples.

Author: Remi Delmas

src/goto-instrument/contracts/dynamic-frames/dfcc_utils.cpp

@@ -105,6 +105,7 @@ const symbolt &dfcc_utilst::create_symbol(
   symbol.is_state_var = true;
   symbol.is_thread_local = true;
   symbol.is_file_local = true;
+  symbol.is_auxiliary = true;
   symbol.is_parameter = is_parameter;
   return symbol;
 }
@@ -134,6 +135,7 @@ const symbolt &dfcc_utilst::create_static_symbol(
   symbol.is_state_var = true;
   symbol.is_thread_local = true;
   symbol.is_file_local = true;
+  symbol.is_auxiliary = true;
   symbol.is_parameter = false;
   return symbol;
 }