epmd: support IPv6 node registration

Allow IPv6 nodes to register with and query epmd. On systems with
IPv6 support:

* epmd listens on both the IPv4 and IPv6 ANY or loopback sockets

* the epmd cli client connects to epmd over the IPv6 loopback

* distributed nodes started with "-proto_dist inet6_tcp" will register
  with epmd over IPv6

The interaction between IPv4 and IPv6 sockets depends on the platform:

* FreeBSD allows multiple "specific" sockets to bind the same port (such
  as 2 sockets listening to the same port on ANY and the loopback).
  Binding port 4369 to IPv4 and IPv6 sockets simulataneously is allowed.

* Linux does not allow the same port to be bound by different sockets.
  Setting the IPV6_V6ONLY socket option is required.

* Windows

  The behaviour differs depending on the version of Windows:

  http://msdn.microsoft.com/en-us/library/windows/desktop/bb513665(v=vs.85).aspx

  According to the site, sockets on Windows XP with Service Pack 1 (SP1)
  and Windows Server 2003 will only listen on either IPv4 or IPv6, so
  creating two sockets is required to service IPv4 and IPv6 traffic on
  the same port. The IPV6_V6ONLY socket option is not supported.

  For Windows Vista and later, a single socket can handle IPv4 and IPv6
  traffic for the same port. The IPV6_V6ONLY socket option is supported
  and is enabled for IPv6 sockets by default.