ndp.c: Turn NDP responder into more declarative implementation

- Add structs for NA, RA, NS, MTU, prefix info, option header,
  link-layer address, RDNSS, DNSSL and link-layer for RA message.

- Turn NA message from purely imperative, going byte by byte,
  to declarative by filling it's struct.

- Turn part of RA message into declarative.

- Move packet_add() to be before the call of ndp() in tap6_handler()
  if the protocol of the packet  is ICMPv6.

- Add a pool of packets as an additional parameter to ndp().

- Check the size of NS packet with packet_get() before sending an NA
  packet.

- Add documentation for the structs.

- Add an enum for NDP option types.

Link: https://bugs.passt.top/show_bug.cgi?id=21
Signed-off-by: AbdAlRahman Gad <abdobngad@gmail.com>
[sbrivio: Minor coding style fixes]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

tap.c

@@ -808,12 +808,13 @@ resume:
 			if (l4len < sizeof(struct icmp6hdr))
 				continue;
 
-			if (ndp(c, (struct icmp6hdr *)l4h, saddr))
+			packet_add(pkt, l4len, l4h);
+
+			if (ndp(c, (struct icmp6hdr *)l4h, saddr, pkt))
 				continue;
 
 			tap_packet_debug(NULL, ip6h, NULL, proto, NULL, 1);
 
-			packet_add(pkt, l4len, l4h);
 			icmp_tap_handler(c, PIF_TAP, AF_INET6,
 					 saddr, daddr, pkt, now);
 			continue;