net: sockets: Add a socket offload module

This patch enables BSD socket offload to a dedicated
TCP/IP offload engine.

This provides a simpler, more direct mechanism than going
through NET_OFFLOAD (zsock -> net_context -> socket conversions)
for those devices which provide complete TCP/IP offload at the
BSD socket level, and whose use cases do not require
IP routing between multiple network interfaces.

To use, configure CONFIG_NET_SOCKETS_OFFLOAD=y, and register
socket_offload_ops with this module.

Fixes #3706

Signed-off-by: Gil Pitney <gil.pitney@linaro.org>