attempt to autodetect when ipv6 is available

Summary:
look for at least one ipv6 address on the system, and if there is
one then enable ipv6 connections. otherwise .. don't.

to help with system where ipv6 is completely disabled.

Test Plan:
Tested locally by turning ipv6 off with:

sysctl -w net.ipv6.conf.default.disable_ipv6=1; sysctl -w net.ipv6.conf.all.disable_ipv6=1

and back on with the opposite commands.

Reviewers: #guam_developers

Differential Revision: https://git.kolab.org/D437