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>

ndp.h

@@ -6,6 +6,7 @@
 #ifndef NDP_H
 #define NDP_H
 
-int ndp(struct ctx *c, const struct icmp6hdr *ih, const struct in6_addr *saddr);
+int ndp(struct ctx *c, const struct icmp6hdr *ih, const struct in6_addr *saddr,
+	const struct pool *p);
 
 #endif /* NDP_H */