[netstack] Add SOL_IP and SOL_IPV6 options to SocketOptions.

We will use SocketOptions for all kinds of options, not just SOL_SOCKET options
because (1) it is consistent with Linux which defines all option variables on
the top level socket struct, (2) avoid code complexity. Appropriate checks
have been added for matching option level to the endpoint type.

Ported the following options to this new utility:
- IP_MULTICAST_LOOP
- IP_RECVTOS
- IPV6_RECVTCLASS
- IP_PKTINFO
- IP_HDRINCL
- IPV6_V6ONLY

Changes in behavior (these are consistent with what Linux does AFAICT):
- Now IP_MULTICAST_LOOP can be set for TCP (earlier it was a noop) but does not
  affect the endpoint itself.
- We can now getsockopt IP_HDRINCL (earlier we would get an error).
- Now we return ErrUnknownProtocolOption if SOL_IP or SOL_IPV6 options are used
  on unix sockets.
- Now we return ErrUnknownProtocolOption if SOL_IPV6 options are used on non
  AF_INET6 endpoints.

This change additionally makes the following modifications:
- Add State() uint32 to commonEndpoint because both tcpip.Endpoint and
  transport.Endpoint interfaces have it. It proves to be quite useful.
- Gets rid of SocketOptionsHandler.IsListening(). It was an anomaly as it was
  not a handler. It is now implemented on netstack itself.
- Gets rid of tcp.endpoint.EndpointInfo and directly embeds
  stack.TransportEndpointInfo. There was an unnecessary level of embedding
  which served no purpose.
- Removes some checks dual_stack_test.go that used the errors from
  GetSockOptBool(tcpip.V6OnlyOption) to confirm some state. This is not
  consistent with the new design and also seemed to be testing the
  implementation instead of behavior.

PiperOrigin-RevId: 344354051

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