Fix wrong tags in the documentation examples

This was silly, but it could cause a lot of confusion.
Add a NO response

Make it possible to send a NO response.
Add a DUMP response type
Change the default response to BAD

If do_respond is called with an unknown state, it should be treated as
an internal server error. This should appear as a BAD response to the
client.
Add a convenience function to send literals

This implementation can only send a sinle literal, but let's review it
when we actually need to prepend something.
Implement READMEM command

This is the most basic command to be used for reading memory data.
Check extraneous arguments to READMEM

Even if all arguments are valid, but there are more arguments, treat it
as a syntax error. The client may be trying to use a later revision of
the protocol without checking.