Dynamic port allocation

Remove the limit of MAX_PORTS ports (default 8) and keep the ports in
a linked list. This allows ptp4l to be used on large machines and in the
future, it will allow dynamic adding and removing of ports while ptp4l is
running.

For this to work, pollfd needs to be dynamically allocated. Changed pollfd
handling from clock_install_fda/clock_remove_fda to notification
(clock_fda_changed), where the clock will rebuild pollfd by querying all its
ports.

Signed-off-by: Jiri Benc <[email protected]>