Since the [main].iwd-config-path functionality, where NM watches for
NMSettingsConnection changes and update IWD network config files with
new settings, has proven to work without issues so far, enable it by
default. Instead of hardcoding /var/lib/iwd as the value, and since the
value can't be probed at NM compile time, query it from IWD's recently-
added D-Bus interface for settings when [main].iwd-config-path is either
missing or set to the new value "auto".
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