Add support for XEP-0174 (XMPP standard)

[sjehuda]

For over a decade, XMPP clients can communicate over LAN and "mesh" by publishing their presence using Avahi (also referred to as "Zeroconf"); see XEP-0174: Serverless Messaging.

Avahi is a system which facilitates service discovery on a local network via the mDNS/DNS-SD protocol suite. This enables you to plug your laptop or computer into a network and instantly be able to view other people who you can chat with, find printers to print to or find files being shared.

Clients with XEP-0174 support: Gajim * and Kopete, and more...

* Support to Gajim will be restored very soon

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See also localsend/localsend#344

[sjehuda]

My best couple of argumenta are:

The benefit to croc, is making this software relevent for many years to come and also benefit from standard features, including encryption means, instead of investing time on any redesigns and extra thinking.

The benefit to both, croc and XMPP users is that they can communicate to each other without the need of having both croc and an XMPP client installed.

[Neustradamus]

@schollz: Any progress on it?

[schollz]

@Neustradamus please feel free to make a PR