main: add tcp_syncookies sanity check
main: add tcp_syncookies sanity check

Check net.ipv4.tcp_syncookies, show error message when it set to 0.
Fixes #1118

Signed-off-by: Takuya ASADA <4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>
Message-Id: <1460738415-3798-1-git-send-email-4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>

kjniemi/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla,duarten/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla