pif: Record originating pif in listening socket refs

For certain socket types, we record in the epoll ref whether they're
sockets in the namespace, or on the host.  We now have the notion of "pif"
to indicate what "place" a socket is associated with, so generalise the
simple one-bit 'ns' to a pif id.

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

passt.h

@@ -35,6 +35,7 @@ union epoll_ref;
 #include <assert.h>
 #include <sys/epoll.h>
 
+#include "pif.h"
 #include "packet.h"
 #include "icmp.h"
 #include "port_fwd.h"