ClientRead: "close" flushes the output buffer

Add a new CommandResult code called "FINISH" which, unlike "CLOSE",
will attempt to flush the output buffer.  This is a one-shot attempt;
it will do one write, and not try again.

src/command/CommandResult.hxx

@@ -47,6 +47,12 @@ enum class CommandResult {
 	 */
 	ERROR,
 
+	/**
+	 * The client has asked MPD to close the connection.  MPD will
+	 * flush the remaining output buffer first.
+	 */
+	FINISH,
+
 	/**
 	 * The connection to this client shall be closed.
 	 */