Fix: Use SOL_SOCKET level for SO_KEEPALIVE on all platform

On Linux, COMPAT_SOCKET_LEVEL was set to SOL_TCP instead
of SOL_SOCKET, this resulted in execution of:
    setsockopt(..., SOL_TCP, TCP_DEFER_ACCEPT, ...)

Instead of:
    setsockopt(..., SOL_SOCKET, SO_KEEPALIVE, ...)

Hence, TCP keep alive was not enabled.

COMPAT_SOCKET_LEVEL is not needed. All platforms supported use the
SOL_SOCKET level for SO_KEEPALIVE.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>