Unix Domain Sockets support for websockets listener

[okisseloff]

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Description

It looks like mosquitto listeners support Unix Domain Sockets to serve mqtt over TCP, while using them with websockets fails.
libwebsockets has a compile option named LWS_UNIX_SOCK that enables UDS support. Suggested change uses the same configuration syntax for websockets listener:

listener 0 /path/to/unix.sock
protocol websockets

but obtains the file name from another unix_socket_path listener property. It also enables UDS support for lws_context instance by appending LWS_SERVER_OPTION_UNIX_SOCK options flag.

Note

I had tried the version of the patch that used LWS_USE_UNIX_SOCK definition to test if libwebsockets support UDS, but it should work only when compiling, while at runtime the dynamic library could be replaced with another instance, so I thought it doesn't make any sense. Haven't found any runtime test for UDS support in libwebsockets.

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