This also corrects the logic of loading config files in m-lua-scripting.
Previously, if an error occured within the iteration function,
the error was not properly propagated to the caller because -EINVAL
was being added to nfiles instead of checked.
The previous approach to loading config files was to ask WP for the
directory and then search those for the config files. This patch changes the
approach - a caller now asks WP to search for a specific config file or
iterate over a config file directory.
This allows us to implement a directory lookup order, i.e.
"wireplumber.conf" may be in XDG_CONFIG_DIR, /etc/,
/usr/share and the first one found is used.
For configuration directories, the new method iterates over all matching
entries (files + directories) and invokes a callback for each entry.
This enables distributions to ship default files in /usr/share/wireplumber
but have admins and users override them on a local basis. For lua scripts in
particular, overriding a distribution-provided file with an empty file
effectively disables it, adding a file adds it in the right sort order.
From the g_file_test documentation:
"For example, (G_FILE_TEST_EXISTS | G_FILE_TEST_IS_DIR) will return TRUE if
the file exists; the check whether it's a directory doesn't matter since the
existence test is TRUE. With the current set of available tests, there's no
point passing in more than one test at a time."
