Enable IPv6 stack by default

The patch enables the use of the IPv6 network stack if available. This
means that dCache can be used with IPv6 sockets.

The JVM still defaults to preferring IPv4 addresses, even when using the
IPv6 stack.

On installation the old behavior can be reactivated by configuring dcache.java.options.extra=-Djava.net.preferIPv4Stack=true.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6901/