udp, passt: Introduce socket packet buffer, avoid getsockname() for UDP

This is in preparation for scatter-gather IO on the UDP receive path:
save a getsockname() syscall by setting a flag if we get the numbering
of all bound sockets in a strict sequence (expected, in practice) and
repurpose the tap buffer to be also a socket receive buffer, passing
it down to protocol handlers.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

util.h

@@ -23,6 +23,7 @@ void debug(const char *format, ...);
 	} while (0)
 
 #define MIN(x, y)		(((x) < (y)) ? (x) : (y))
+#define MAX(x, y)		(((x) > (y)) ? (x) : (y))
 
 #define IN_INTERVAL(a, b, x)	((x) >= (a) && (x) <= (b))