server: destroy the socket event source on display destroy

On wl_display_add_socket(), the listening socket fd is added to the
event loop. However, wl_event_source object is not stored and hence
cannot be freed, resulting in a minor leak.

Store wl_event_source pointer in struct wl_socket so we can track it,
and destroy it on wl_display_destroy(). The event loop itself must be
destroyed after destroying the event sources linked to it.

Fixes a Valgrind reported memory leak.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>