1. Add special option where an empty host means 'wildcard for IPv4 and/or IPv6'
which means ':1234' can be used to set only the portnum.
2. Only add this protocol wildcard if --autolisten=1 (default)
and no other addresses specified.
3. Pass it down to gossipd, so it can handle errors correctly: in most cases,
it's fatal not to be able to bind to a port, but for this case, it's OK
if we can only bind to one of IPv4/v6 (fatal iff neither).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Rusty Russell
52917ff6c9