Use /proc/net/sockstat6 to detect IPv6 support

The file /sys/module/ipv6 does not exist in all container
implementations (e.g. Virtuozzo). Using /proc/net/sockstat6
detects IPv6 support reliably in these environments, too.

This file does not exist when the kernel is not compiled with
IPv6 support, or if IPv6 support is disabled, so simply checking
for existence should be a suitable check.

Fixes #2059