MEDIUM: connections: Introduce a handshake pseudo-XPRT.

Add a new XPRT that is used when using non-SSL handshakes, such as proxy
protocol or Netscaler, instead of taking care of it in conn_fd_handler().
This XPRT is installed when any of those is used, and it removes itself once
the handshake is done.
This should allow us to remove the distinction between CO_FL_SOCK* and
CO_FL_XPRT*.
diff --git a/include/types/connection.h b/include/types/connection.h
index 74060b0..e706b87 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -301,6 +301,7 @@
 enum {
 	XPRT_RAW = 0,
 	XPRT_SSL = 1,
+	XPRT_HANDSHAKE = 2,
 	XPRT_ENTRIES /* must be last one */
 };