Add eventpoll patch

Add eventpoll patch

The polling interface used by posix-socket is based on the eventpoll
API. This patch modifies lwip to call a custom callback for event
notification in lwip's posix-socket driver. It also changes definitions
in lwip's internal socket.h to be compatible with external definitions
we use.

Signed-off-by: Marc Rittinghaus <marc.rittinghaus@kit.edu>
Reviewed-by: Cezar Craciunoiu <cezar.craciunoiu@gmail.com>
Reviewed-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Approved-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: #17