Check API on both ipv4 and ipfv6

[lidel]

Fun edge case that is super painful to debug:

i had a lingering service bound to: [::1]:8080 (ipv6), so the browser was trying that first. go-ipfs does not bind to that for the gateway so i wasn't getting any feedback ipfs companion could also check for both -- as localhost in the browser will try ipv6 first

Needs more analysis, but initial thoughts:

• enforce use of raw IP for API port (if it does not yet), convert localhost if needed (removes ambiguity which v is used)
• support entering API URL as ip4 and ip6 (needs fix for #904)
• when one fails, try another one, and fixup URL if second one is successful (test with /api/v0/id)