security: Introduce unsetKeyLocator method into SignatureInfo and Signature

Some specific signature types also provide an advisory (non-virtual)
override for unsetKeyLocator that disables reset and throws an
Exception.  As before, if an application really wants, it still can
unset the KeyLocator by setting a new SignatureInfo without the
KeyLocator.

Change-Id: Ibefd5f9ffbcb91f8a83e76db8704cba1b984f914
Refs: #1750
diff --git a/src/security/signature-sha256-with-ecdsa.hpp b/src/security/signature-sha256-with-ecdsa.hpp
index a20e6b0..4b0eb7f 100644
--- a/src/security/signature-sha256-with-ecdsa.hpp
+++ b/src/security/signature-sha256-with-ecdsa.hpp
@@ -48,6 +48,9 @@
   explicit
   SignatureSha256WithEcdsa(const Signature& signature);
 
+private:
+  void
+  unsetKeyLocator();
 };
 
 } // namespace ndn