feature: each proxy ipv6 settings and auto-set-ipv6

[patterniha]

some proxies support ipv6, and some don't.

because we only have one global "prefer ipv6" setting, we have to constantly turn the "prefer ipv6" off and on for each config.

we need "prefer ipv6" setting for each config.

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I suggest add "auto-set-ipv6" too:

like "http-test" add "ipv6-test":

sending a http request to https://ip6only.me.
(because dns query result for ip6only.me only has ipv6.)
and if connection fail, turn off the "prefer ipv6" for that config, else turn on.

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"prefer ipv6" should be renamed to "enable ipv6", because it actually enable/disable the ipv6.