MINOR: session: simplify error path in session_accept_fd()

MINOR: session: simplify error path in session_accept_fd()

Now that this function is always called with an initialized connection
and that the control layer is always initialized, we don't need to play
games with fdtab[] to decide how to close, we can simply rely on the
regular close path using conn_ctrl_close(), which can be fused with
conn_xprt_close() into conn_full_close().

The code is cleaner because the FD is now used only for some
protocol-specific setup (that will eventually have to move) and to
try to send a hard-coded HTTP 500 error message on raw sockets.