If no setting name is found in the file, it means that the file
possibly contains a setting superclass (e.g. NMSettingIPConfig)
without any property definition; just ignore it.

That serves no purpose whatsoever.

It makes little sense to produce a YAML and then decompose it.

Add "---dbus---" sections to the NMSetting property docs, in the same
style as the plugin docs, parse them out into a file
"nm-setting-docs-overrides.xml", and use them to override the GObject
property docs in nm-setting-docs.xml.

This lets us put more D-Bus-specific information in the setting docs,
without cluttering up the property docs, and it also lets us document
dbus-only properties.

Move the settings/plugins doc generation from libnm-util to
libnm-core, since libnm-util isn't being updated for all new
properties.

With this commit, the keyfile and ifcfg-rh documentation is basically
unchanged, except that deprecated properties are now gone, and new
properties have been added, and the sections are in a different order.
(generate-plugin-docs.pl just outputs the settings in Makefile order,
and they were unsorted in libnm-util, but are sorted in libnm-core).

The settings documentation used for nm-settings.5, the D-Bus API docs,
and the nmcli help is changed a bit more at this point, and mostly for
the worse, since the libnm-core setting properties don't match up with
the D-Bus API as well as the libnm-util ones do. To be fixed...

(I also removed the "plugins docs" line in each plugin docs comment
block while moving them, since those blocks will be used for more than
just plugins soon, and it's sort of obvious anyway.)