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/OtherCommands.cxx

@@ -99,7 +99,7 @@ CommandResult
 handle_close(gcc_unused Client &client,
 	     gcc_unused int argc, gcc_unused char *argv[])
 {
-	return CommandResult::CLOSE;
+	return CommandResult::FINISH;
 }
 
 CommandResult