udp: Connection tracking for ephemeral, local ports, and related fixes

As we support UDP forwarding for packets that are sent to local
ports, we actually need some kind of connection tracking for UDP.
While at it, this commit introduces a number of vaguely related fixes
for issues observed while trying this out. In detail:

- implement an explicit, albeit minimalistic, connection tracking
  for UDP, to allow usage of ephemeral ports by the guest and by
  the host at the same time, by binding them dynamically as needed,
  and to allow mapping address changes for packets with a loopback
  address as destination

- set the guest MAC address whenever we receive a packet from tap
  instead of waiting for an ARP request, and set it to broadcast on
  start, otherwise DHCPv6 might not work if all DHCPv6 requests time
  out before the guest starts talking IPv4

- split context IPv6 address into address we assign, global or site
  address seen on tap, and link-local address seen on tap, and make
  sure we use the addresses we've seen as destination (link-local
  choice depends on source address). Similarly, for IPv4, split into
  address we assign and address we observe, and use the address we
  observe as destination

- introduce a clock_gettime() syscall right after epoll_wait() wakes
  up, so that we can remove all the other ones and pass the current
  timestamp to tap and socket handlers -- this is additionally needed
  by UDP to time out bindings to ephemeral ports and mappings between
  loopback address and a local address

- rename sock_l4_add() to sock_l4(), no semantic changes intended

- include <arpa/inet.h> in passt.c before kernel headers so that we
  can use <netinet/in.h> macros to check IPv6 address types, and
  remove a duplicate <linux/ip.h> inclusion

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

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