(main): Add --noipv6 option, and if set do not bind to an ipv6 port
On at least one system I am using (OpenBSD) the 1.2.0 version of
svnserve detects there is an ipv6 network, and binds to that network
ONLY. So there is no listening port setup on ipv4, so no ipv4 clients
As I do not use ipv6 I added an option --noipv6 which will force it to
bind to the ipv4 network as previous versions did.
Others with both ipv6 and ipv4 on their systems may encounter similar
problems as I do not think this is specific to OpenBSD.
Ideally the fix should actually allow it to bind to both ipv6 and ipv4
networks, however that fix is beyond me at the moment.