gRPC: Avoid AF_INET6 sockets when the ::1 loopback address doesn't exist.

On Linux with disable_ipv6=1, we can create sockets bound to [::]:port, yet
connecting to that address triggers an Unreachable error.  Since IPv6 is
useless on such machines, it's cleaner to turn it off than expose users to a
half-broken state.
	Change on 2014/12/17 by pmarks <pmarks@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82387437