Support for unix domain socket based connection passing

[qubeck]

This feature adds support for unix domain socket based connection passing to allow an other process to pass already accepted connections to a running instance of mosquitto broker.

In case a unix domain socket listener is being used this patch evaluates additionally the first incoming message for presence of ancillary data of type SCM_RIGHTS to take a passed connection file descriptor and use it instead of the original accepted socket.

(This pull request makes #771 obsolete)

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[karlp]

You're going to have to (at least) update the makefile too, as they're canonical for non-windows systems, not cmake

[qubeck]

You're going to have to (at least) update the makefile too, as they're canonical for non-windows systems, not cmake

Thx, I'll provide integration to the non-cmake based makefile build