Abolish playrtp's "readahead" variable (and associated -b option).

There's just minbuffer and maxbuffer now.  minbuffer has the same
meaning as before but is additionally used for what readahead used to
be.  maxbuffer defaults to 2*minbuffer.

The man page mentions them, but refers you to the source code, which
now goes into more detail in comment than it did before.

playrtp also no longer adjusts the socket buffer size, accepting the
operating system's default.  Even the relatively small (~40K) Mac one
gives us a couple of dozen typical-sized packets; experimentally it's
quite enough to cope even with a 'make -j' of DisOrder running on the
same host.