Get rid of need for java.net.preferIPv4Stack

New address picking scheme:

• First we detect which addresses are available: IPv4 and/or IPv6 (dual-stack if both)
• When we encounter an IPv4 address:
  • If dual-stack or IPv4 stack: use it
  • If IPv6-only stack: use it unless it is a class D (multicast) address: then convert it to an IPv6-mapped address
• When we encounter an IPv6 address:
  • If dual-stack or IPv6-only stack: use it
  • Else: throw an exception (IPv6 address in an IPv4-only stack)