prosody.cfg.lua.dist: Remove network_backend option
The only sensible option for most people these days is "epoll". The option is
still supported, but it should only be needed by packagers for specific platforms
(e.g. BSD may choose libevent for kqueue) or other special cases.
Welcome hackers!
This project accepts and *encourages* contributions. If you would like to get
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information on these at https://prosody.im/discuss
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Have fun :)