New debugging program: rtpmon. This listens for packets the same way

New debugging program: rtpmon.  This listens for packets the same way
disorder-playrtp does (and ought to be de-duped against it), and
periodically displays the sample rate over various intervals.

As the code stands the longest-term column is consistently showing
44105Hz, which since it's faster than the rate the sound is played at
will eventually fill up playrtp's (and the kernel's) buffers and force
packets to be dropped, affecting playback quality.