udp: Introduce recvmmsg()/sendmmsg(), zero-copy path from socket

Packets are received directly onto pre-cooked, static buffers
for IPv4 (with partial checksum pre-calculation) and IPv6 frames,
with pre-filled Ethernet addresses and, partially, IP headers,
and sent out from the same buffers with sendmmsg(), for both
passt and pasta (non-local traffic only) modes.

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

passt.h

@@ -134,3 +134,6 @@ struct ctx {
 	struct udp_ctx udp;
 	struct icmp_ctx icmp;
 };
+
+void proto_update_l2_buf(unsigned char *eth_d, unsigned char *eth_s,
+			 uint32_t *ip_da);