MEDIUM: connection: use CO_FL_WAIT_XPRT more consistently than L4/L6/HANDSHAKE

MEDIUM: connection: use CO_FL_WAIT_XPRT more consistently than L4/L6/HANDSHAKE

As mentioned in commit c192b0ab95 ("MEDIUM: connection: remove
CO_FL_CONNECTED and only rely on CO_FL_WAIT_*"), there is a lack of
consistency on which flags are checked among L4/L6/HANDSHAKE depending
on the code areas. A number of sample fetch functions only check for
L4L6 to report MAY_CHANGE, some places only check for HANDSHAKE and
many check both L4L6 and HANDSHAKE.

This patch starts to make all of this more consistent by introducing a
new mask CO_FL_WAIT_XPRT which is the union of L4/L6/HANDSHAKE and
reports whether the transport layer is ready or not.

All inconsistent call places were updated to rely on this one each time
the goal was to check for the readiness of the transport layer.