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/signature-info.hpp b/src/signature-info.hpp
index c0a2d88..4ef60fc 100644
--- a/src/signature-info.hpp
+++ b/src/signature-info.hpp
@@ -78,6 +78,10 @@
   void
   setKeyLocator(const KeyLocator& keyLocator);
 
+  /// @brief Unset KeyLocator
+  void
+  unsetKeyLocator();
+
   /**
    * @brief Get KeyLocator
    *