Add GATEWAY_IP6 variable

[DanWin]

This pull request adds an IPv6 gateway configuration variable

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[ArrayBolt3]

Everything here looks correct to me. Will try to work on testing this and the other PRs soon.

[ArrayBolt3]

This slightly scares me because IPv6 addresses have [, ], and : characters in them, all of which can be treated specially by the shell in some contexts. Could we use some extra quotes to make sure nothing goes awry here? (The GATEWAY_IP variable doesn't strictly need them, but maybe we could add them there for consistency too.)

Suggested change

	GATEWAY_IP=\"$GATEWAY_IP\"
	GATEWAY_IP6=\"$GATEWAY_IP6\"
	GATEWAY_IP="\"$GATEWAY_IP\""
	GATEWAY_IP6="\"$GATEWAY_IP6\""