handle ipv6 addresses suffixed with scope id

[staltz]

Related to #51, I get a lot of errors that Manyverse is crashing with this:

Error: listen EINVAL fe80::c376:4a3f:a128:3466:26831
    at Server.setupListenHandle [as _listen2] (net.js:1269:19)
    at listenInCluster (net.js:1334:12)
    at doListen (net.js:1460:7)
    at process._tickCallback (internal/process/next_tick.js:63:19)

And it comes from this multiserver's net plugin. After a lot of investigation, I figured out why that IPv6 address is "invalid": all of these IPv6 addresses that I got reports from were all link-local addresses, and given multiple network interfaces on the device, the IPv6 addresses needs a suffix to disambiguate which network interface is that address related to. See IETF RFC 6874.

So to fix those kind of crashes, I just need to provide the zone id %wlan0 or %en0 (or whatever) as a suffix on the IP address, so if we call server.listen(port, host) using the host shown below, then the crash is fixed!

-host = 'fe80::c376:4a3f:a128:3466'
+host = 'fe80::c376:4a3f:a128:3466%wlan0'

Read more: https://superuser.com/questions/99746/why-is-there-a-percent-sign-in-the-ipv6-address

Given the IPv6 Link-Local addressing, where every interface has the same network (fe80::/10), there must be some way to distinguish which specific network is meant when referring to a link-local address. That is what the Scope ID, also called a Zone ID so as not to be confused with the multicast scope flags, is for.

What this PR does, however, is to hide the %wlan0 for remote peers, because the RFC says:

URIs including a ZoneID are to be interpreted only in the context of the host at which they originate, since the ZoneID is of local significance only.

That's why I added some code in stringify() to remove the zone ID before sharing it with peers.

[arj03]

Looks good

[dominictarr]

merged into 3.4.0