core: add support for RFC7217 stable privacy addressing

RFC7217 introduces an alternative mechanism for creating addresses during
stateless IPv6 address configuration. It's supposed to create addresses whose
host part stays stable in a particular network but changes when the hosts
enters another network to mitigate possibility of tracking the host movement.

It can be used alongside RFC 4941 privacy extensions (temporary addresses)
and replaces the use of RFC 4862 interface identifiers.

The address creation mode is controlld by ip6.addr_gen_mode property
(ADDR_GEN_MODE in ifcfg-rh), with values of "stable-privacy" and "eui-64",
defaulting to "eui-64" if unspecified.

The host part of an address is computed by hashing a system-specific secret
salted with various stable values that identify the connection with a secure
hash algorithm:

  RID = F(Prefix, Net_Iface, Network_ID, DAD_Counter, secret_key)

For NetworkManager we use these parameters:

* F()
  SHA256 hash function.

* Prefix
  This is a network part of the /64 address

* Net_Iface
  We use the interface name (e.g. "eth0"). This ensures the address won't
  change with the change of interface hardware.

* Network_ID
  We use the connection UUID here. This ensures the salt is different for
  wireless networks with a different SSID as suggested by RFC7217.

* DAD_Counter
  A per-address counter that increases with each DAD failure.

* secret_key
  We store the secret key in /var/lib/NetworkManager/secret_key. If it's
  shorter than 128 bits then it's rejected. If the file is not present we
  initialize it by fetching 256 pseudo-random bits from /dev/urandom on
  first use.

Duplicate address detection uses IDGEN_RETRIES = 3 and does not utilize the
IDGEN_DELAY delay (despite it SHOULD). This is for ease of implementation
and may change in future. Neither parameter is currently configurable.

src/NetworkManagerUtils.h

@@ -286,6 +286,12 @@ gboolean nm_utils_get_ipv6_interface_identifier (NMLinkType link_type,
 void nm_utils_ipv6_addr_set_interface_identfier (struct in6_addr *addr,
                                                  const NMUtilsIPv6IfaceId iid);
 
+gboolean nm_utils_ipv6_addr_set_stable_privacy (struct in6_addr *addr,
+                                                const char *ifname,
+                                                const char *uuid,
+                                                guint dad_counter,
+                                                GError **error);
+
 void nm_utils_ipv6_interface_identfier_get_from_addr (NMUtilsIPv6IfaceId *iid,
                                                       const struct in6_addr *addr);