prosody: support Lua 5.4 and improve security

The changes here are:

* prosody can be built with Lua 5.4 (as recommended since the 0.12
  series), also thanks to lua-unbound being available for Lua 5.4
* the prosody user's home directory is back to the default /nonexistent
  (prosody finds its own data directory nonetheless, as it is a
  compile-time option)
* the corresponding directories created (data directory, PID directory,
  logging) do not seem to actually require write access (or not anymore)

These last two changes together get rid of the security report "user
prosody home directory is group writable" from the daily insecurity
checks on NetBSD.

Tested on NetBSD/amd64.

(khorben)