message: add default ctor

Useful for stuffing messages in async-queues

lib/message/mu-message.cc

@@ -104,6 +104,10 @@ get_statbuf(const std::string& path, Message::Options opts = Message::Options::N
 	return Ok(std::move(statbuf));
 }
 
+Message::Message() noexcept:
+	priv_{std::make_unique<Private>(Message::Options::None)}
+{}
+
 
 Message::Message(const std::string& path,  Message::Options opts):
 	priv_{std::make_unique<Private>(opts)}

lib/message/mu-message.hh

@@ -55,6 +55,12 @@ public:
 						 * CJK and other languages. */
 	};
 
+	/**
+	 * Default CTOR; not useful by itself, but can be moved into.
+	 */
+	Message() noexcept;
+
+
 	/**
 	 * Move CTOR
 	 *