Rename send_reply_header_callback

We weren't using it as a callback most of the time, so for the avoidance
of confusion, stop calling it that. Instead, call it send_reply_header,
which is still what it does -- just not as a callback. Usually.
Implement flushing
Remove dead code
Guard against incorrect usuage slightly better
Add missing semicolon
Use correct variable name in macro line
Don't forget to update the offsets
Make debug output somewhat more useful...

... for developers. Significantly less useful for users.

We should have a debug level, really. Not for now.