Add updater for dbus introspection in man pages

Compares to gdbus output, the values of properties are replaced by ellipses.
For arrays and strings, the outer markers are kept. This is obviously also told
by the type string, but it seems a bit easier to read this way.

For any elements which are undocumented, a comment is inserted in sources.
"Undocumented" means that the expected element was not found. This might
require some adjustments if I missed some markup types.

Invocation is manual:
$ tools/update-dbus-docs.py tools/update-dbus-docs.py man/org.freedesktop.login1.xml
$ tools/update-dbus-docs.py tools/update-dbus-docs.py man/org.freedesktop.resolve1.xml
$ tools/update-dbus-docs.py tools/update-dbus-docs.py man/org.freedesktop.systemd1.xml
...

If some object is not found on the bus, the existing output is retained. So the
user needs to make sure that the appropriate objects have been instantiated
before calling this. We don't change the dbus interface very often, so I think
this manual mode is OK as a starting point. Making this fully automatic later
would be nice of course.