ENH: cleanup Pstream internal names, cull unneeded parameters

- Pstreams can be ascii/binary but are always currentVersion

- rename UIPstream externalBuf_ to 'recvBuf_' for similar naming as
  PstreamBuffers and symmetry with UOPstream::sendBuf_

- specific enum size for commsType (for more compact structures in the
  future). Default construct lists items.

BUG: possible incidental indexing in UIPstream::read(char*, std::streamsize)

- raw reading had been split into beginRawRead(), readRaw().
  However, this could change the current input position (due to word
  boundary alignment), even if the expected count is zero. Make a
  no-op for count == 0. This mirrors UOPstream::write behaviour.