[plt-scheme] IPv6 and Linux (was: web-server and IPv6?)

I've changed `tcp-listen' for Linux one more time, so let me know if
you see any problems.

Details below...

At Fri, 11 Nov 2005 12:55:52 -0700, Matthew Flatt wrote:
> My solutions:
> 
>  1. For Linux 2.4.20 and earlier, `tcp-listen' uses only IPv4 mappings.
> 
>  2. For Linux 2.4.21 and later, `tcp-listen' uses only IPv4 mappings
>     when no hostname is supplied.
> 
>  3. For Linux 2.4.21 and later, when `tcp-listen' creates an IPv6
>     listener (because a given hostname maps to an IPv6 address), the
>     listener accepts only IPv6 connections.

This biggest problem with this approach is that it relies on
compile-time detection of the kernel version.

In the new approach, `tcp-listen' takes special action only when the
listener has both IPv4 and IPv6 addresses:

 * If IPv6 sockets can't be created due to EAFNOSUPPORT, or if the
   IPV6_V6ONLY socket option doesn't work (which means Linux 2.4.20 and
   earlier), then the IPv6 addresses are silently(!!!) ignored.

 * Otherwise, IPv6 sockets for the listener accept only IPv6
   connections --- though you normally can't tell, since having both
   IPv4 and IPv6 addresses probably means that there's an IPv4 address
   for the same interface.

Of course, I don't like silently ignoring IPv6 addresses, but it seems
like a better compromise than the old one. These rules do not apply you
specify an IPv6-only address like "::1", so if that fails, then you see
the error.

Matthew