poll: Make handling of fds array actually dynamic

poll: Make handling of fds array actually dynamic

Previously we relied on the max_fds argument to register_for_poll
being big enough and allocated an array at startup.  But we are going
to want to be more dynamic, so actually do the dynamic array resizing.

We now start with a zero-sized array and increase it as needed.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>