CA-105004: Enable SO_KEEPALIVE for stunnel server

We should also consider fine tweaking KEEPALIVE parameters when we upgrade to the new version of stunnel.

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
Merge pull request #1274 from zli/xenserver/CA-105004

CA-105004: Enable SO_KEEPALIVE for stunnel server