Explicitly disable dumping core (closes #72)

This is a precaution to avoid saving sensitive data to disk, e.g. session keys.
We expect that corefiles are not world readable, but they're still sitting on
the physical disk and it's safer just to disable creating them.

GitHub issue #71 deals with a similar concern.

src/crypto/crypto.h

@@ -84,6 +84,8 @@ namespace Crypto {
     Session( const Session & );
     Session & operator=( const Session & );
   };
+
+  void disable_dumping_core( void );
 }
 
 #endif