Core: handle getsockopt(TCP_FASTOPEN) failures.

Linux returns EOPNOTSUPP for non-TCP sockets and ENOPROTOOPT for TCP
sockets, because getsockopt(TCP_FASTOPEN) is not implemented so far.

While there, lower the log level from ALERT to NOTICE to match other
getsockopt() failures.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>