Split af_local.cc into four files

The source file af_local.cc implemented both pipes and bi-directional
pipes (unix domain stream socketpair), using a common buffer implemetation.

As suggested by Guy, split this file into four files:

pipe_buffer.cc and pipe_buffer.hh contain the common buffer implementation,
class pipe_buffer. Since this buffer basically implements a single-direction
pipe, I renamed it from "af_local_buffer" to pipe_buffer.

af_local.cc now contains just the unix domain stream socketpair
implementation, implemented using two pipe_buffer objects.

af_pipe.cc contains the Posix pipe() implementation, implemented using
one pipe_buffer object..