P2 and P3 subprocess stdout seem to have very odd/different behaviours
in the face of mixing select and buffering.
Use raw/low-level IO (os.read) with select to try and fix that
up. Also read more than one byte at a time since os.read will not
block *if* the fd is readable, it will just get whatever data is
immediately available.
Xavier Morel
8361ec8819