various: Use 0.12+ API for serving files from the file system over HTTP Using mod_http_files this way is deprecated. Having the base behavior in a separate module makes it easier to have a specialized module with coherent behavior that does not do things when not explicitly enabled.

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.