udp: Split splice field in udp_epoll_ref into (mostly) independent bits

The @splice field in union udp_epoll_ref can have a number of values for
different types of "spliced" packet flows.  Split it into several single
bit fields with more or less independent meanings.  The new @splice field
is just a boolean indicating whether the socket is associated with a
spliced flow, making it identical to the @splice fiend in tcp_epoll_ref.

The new bit @orig, indicates whether this is a socket which can originate
new udp packet flows (created with -u or -U) or a socket created on the
fly to handle reply socket.  @ns indicates whether the socket lives in the
init namespace or the pasta namespace.

Making these bits more orthogonal to each other will simplify some future
cleanups.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

passt.h

@@ -31,6 +31,8 @@ struct tap_l4_msg {
 
 union epoll_ref;
 
+#include <stdbool.h>
+
 #include "packet.h"
 #include "icmp.h"
 #include "port_fwd.h"