MAJOR: checks: make use of the connection layer to send checks

MAJOR: checks: make use of the connection layer to send checks

This is a first step, we now use the connection layer without the data
layers (send/recv are still used by hand). The connection is established
using tcp_connect_server() and raw_sock is assumed and forced for now.

fdtab is not manipulated anymore and polling is managed via the connection
layer.

It becomes quite clear that the server needs a second ->ctrl and ->xprt
dedicated to the checks.