We've never really cared how many times the ring buffer has become
empty, how many times data was added to it, or how many times we've
been over the highwater mark.  These have effectively always been
event signalling, with extra hackiness to clear "extra" semaphore
posts.

This commit removes RINGBUF_SEM entirely, and uses events for
everything.  There's an empty event (set when the ring buffer is empty),
a data event (set when the ring buffer is not empty), and a highwater
event (set when the ring buffer has at least highwater mark bytes).

A RingBufWrite() will set data and highwater events if applicable,
and clear the empty event.

A RingBufRead() will set empty event and clear data and highwater
events if applicable.

RingBufReInit() will now set the empty event, and clear the data
and highwater events.

These are the only actions the RingBuf API will perform on the
events, it does *not* clear the highwater event on a RingBufWrite()
if the buffer does not have enough bytes in it for example, this
will *only* be done by RingBufRead() or RingBufReInit().  This
allows consumers to force specific behaviours (such as forcing
the highwater event on shutdown to prevent waiting for it).  Since
the current code was able to deal with the semaphores having
arbitrarily high counts, the existing code shouldn't have any
issue with this.

For things that want to shut something down, we're now setting both
the data and highwater events to ensure it doesn't wait for a
timeout (even for things that don't use highwater like the inbuf).
The RingBuffer API should grow a thing to do this (RingBufferFlush
perhaps?) rather than the consumers poking into the innards
arbitrarily.

Tested on the webserver and RLogin server and seems to be working
great.  sexyz builds.  It seems there's some Windows thing that
uses the ringbuffer sem for something, but I couldn't figure out
what... updated the build file, but no clue if that's broken or not
now... I'm sure DigitalMan will be happy to fix it if CI doesn't
catch it. ;)

This should allow the console object to grow a "flush" method that
will set the highwater and data events.