Fixes: https://github.com/ngtcp2/ngtcp2/issues/261

Fixes: https://github.com/ngtcp2/ngtcp2/issues/257

Fix handling of "*" as address in example server

Fix Windows build

Use winsock.h on windows instead of sys/socket.h

Without these changes, library was failing to build on both Windows and
FreeBSD.

remove redundant return

Remove conn and user_data from ngtcp2_rand in order to use it before
conn is initialized.