Check several possible standard paths when there is no path to the
configuration file given as argument. To find the configuration file
following paths are used. The local directory `./config.toml`, the
user global configuration directory
`$XDG_CONFIG_HOME/whakarite/config.toml` and the system wide
configuration directory `/etc/whakarite/config.toml`. Note that in the
later two cases the directory which contains the `config.toml`
receives its name from the `$CARGO_BIN_NAME` environment variable at
compile time.
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