Fix waiting poll on unix-domain socketpair

If poll() was waiting on a file descriptor from socketpair_af_local, we
would never wake it up, and an example of this is the failure in a
recently committed fix to tst-af-local.cc.

The problem is that when one writes to one end of the socket, we need to
call wake_poll() on the other end of the socket, so we need to remember
which "struct file *" is attached to each end of the af_local_buffer objects.

What I did is what I thought the most elegant solution is:

Rather than having "sender" and "receiver" of af_local_buffer booleans,
they are now "struct file *". I added new functions, attach_sender(f) and
attach_receiver(f), which set the file* we'll need to notify for each
end; These functions are analogous to functions detach_sender, detach_receiver
that we already had.

After each interesting event - read, write, close, etc - we notify the
appropriate file*, using poll_wake.

attach_sender(f) and attach_receiver(f) is called by af_local_init(f) - which
used to be empty and now does something. Note how af_local_init(f) only
does send->attach_sender(f) and receive->attach_receiver(f), but doesn't
touch the two others (send->attach_receiver, receive->attach_sender) -
these other two are set when the second file descriptor, with the send
and receive fifos in reversed roles, is initialized with its af_local_init.

After this fix, the new af_local_test works correctly.