Fix #3672: Improve javalib java.net handling of NetworkInterface indices

[LeeTibbert]

Fix #3672

Most end users of java.net use IPv6 global addresses. This PR is for the few who use link-local addresses.

javalib java.net methods should now handle IPv6 sin6_scope_ids, known to Java as NetworkInterface indices
so that a connection using an IPv6 link-local (FE80::) address should be accepted on by a server yielding
an IPv6Address which can be used to connect back over the local link. That is, a round trip using
link-local addresses should work.

[LeeTibbert]

I am marking this as WIP & Draft because I did some more private testing.

Looks like accept() on a link-local address is not working at least on macOS.

I have not tried it yet with a macOS client connecting to a Linux server.

[LeeTibbert]

I believe this PR is ready for review, in the fullness of time.

Whilst trying to increase my confidence in the changes of this PR, I discovered & reported
Issues #3705 and #3706.

When I hacked around those in my private sandbox, these changes allowed a send/receive "round trip"
over an IPv6 local link. Debug printouts of the addresses involved indicated that lPv6 link-local addresses,
with the expected NetworkInterface indices (sin6_scope_id) were being used.

I examined the build logs to see if this PR introduced any new errors. Did not see such.