log:

    Fix an old bug in poll().  When a signal is handled while we're
    blocked in select(), this will raise select.error with errno set
    to EINTR.  The except clauses correctly ignores this error, but
    the rest of the logic will then call read() for all objects in
    select's *input* list of read file descriptors.  Then when an
    object's read_handler() is naive, it will call recv() on its
    socket, which will raise an IOError, and then asyncore decides to
    close the socket.  To fix this, we simply return in this case.

Rather than incorporating a fixed copy of asyncore.py in Zope2 and
ZODB3, I'm incorporating a fixed copy of just asyncore.poll() in this
file, since we already have a copy of asyncore.loop() here.