Add support for Unix socket

[nfsnfs]

Add support to use unix socket as mqtt channel.

[FlorianLudwig]

Hi @nfsnfs

hbmqtt has been deprecated by it's author. We created a fork here: https://github.com/Yakifo/amqtt

I like this change and would like to include it in our fork.

Would you be interested to create your MR there as well?

Also, would you be willing to write a test for it?

Thanks :)

[nfsnfs]

Hi @FlorianLudwig ,

Got it. I will find some time to send this PR (including test) to amqtt.

Thanks!