Add IPv6 support

new cli options (RPC ones also apply to wallet):
  --p2p-bind-ipv6-address (default = "::")
  --p2p-bind-port-ipv6    (default same as ipv4 port for given nettype)
  --rpc-bind-ipv6-address (default = "::1")

  --p2p-use-ipv6          (default false)
  --rpc-use-ipv6          (default false)

  --p2p-require-ipv4      (default true, if ipv4 bind fails and this is
                           true, will not continue even if ipv6 bind
                           successful)
  --rpc-require-ipv4      (default true, description as above)

ipv6 addresses are to be specified as "[xx:xx:xx::xx:xx]:port" except
in the cases of the cli args for bind address.  For those the square
braces can be omitted.

contrib/epee/include/net/local_ip.h

@@ -27,10 +27,38 @@
 
 #pragma once
 
+#include <string>
+#include <boost/algorithm/string/predicate.hpp>
+#include <boost/asio/ip/address_v6.hpp>
+
 namespace epee
 {
   namespace net_utils
   {
+
+    inline
+    bool is_ipv6_local(const std::string& ip)
+    {
+      auto addr = boost::asio::ip::make_address_v6(ip);
+
+      // ipv6 link-local unicast addresses are fe80::/10
+      bool is_link_local = addr.is_link_local();
+
+      auto addr_bytes = addr.to_bytes();
+
+      // ipv6 unique local unicast addresses start with fc00::/7 -- (fcXX or fdXX)
+      bool is_unique_local_unicast = (addr_bytes[0] == 0xfc || addr_bytes[0] == 0xfd);
+
+      return is_link_local || is_unique_local_unicast;
+    }
+
+    inline
+    bool is_ipv6_loopback(const std::string& ip)
+    {
+      // ipv6 loopback is ::1
+      return boost::asio::ip::address_v6::from_string(ip).is_loopback();
+    }
+
     inline
     bool is_ip_local(uint32_t ip)
     {