Basic split-stack functionality

Basic split-stack functionality

Enable split-stack detection by default.

There is now enough split-stack support to enable accepting IPv6 client
connections on systems with separate IPv4/IPv6 stacks. Also some limited
server connection capabilities (see tcp_outgoing_addr config hacks).

SNMP, ICP, HTCP listeners and outbound connections currently default to
IPv4-only on these systems to retain backward-compatibility.
But may be explicity configured to IPv6-only. There is no support as yet
for dual-protocol behaviour in the sockets of these three protocols.