It just feels nicer to be explicit about the filenames and
not rely on a specific naming.

Also, in meson we can directly pass the target as argument, which
expands to the filename but also adds a dependency.

Found by lgtm.com.

We also need to build with python2. No f-strings.

Fixes: 8fc7b6df12ed ('tools: rework generating documentation from libnm meta data')

"nm-setting-ip-config.c" is a base class for IPv4 and IPv6 settings. So
far, any tags there were ignored, which was not obvious. It can be
useful to document common properties there. Well, maybe every property
better has a IPv4/IPv6 specific text, but that should not be a technical
limitation of the tool. So also honor the base file for "ipv4" and
"ipv6" settings.

When doing that, the settings are no longer processed in the order as
they are provided on the command line. Because, one file would be parsed
twice, it wouldn't make much sense. Instead, sort the my setting name.
The advantage is that the generated XML is independent from the order
that make/meson passes to the tool.

If the information is missing, the entire attribute should not be there.
Don't set it to the empty word.

Also, don't alias the "variable" attribute to the "name". It's not clear
what the "variable" fields is supposed to mean, but if it's not
explicitly set, don't make up the information. If a user of that
information cares, the can always fallback to the "name".

This generates '<setting name="XXX">\n</setting>' nodes in case there is
no additional data. Don't do that. Just '<setting name="XXX"/>'

Our docs can be long. It's important to be able to express paragraphs.
Honor a blank line to include a newline. For XML often whitespace is
ignored, but our tools can choose to honor the newline.

Also, don't strip the whitespace from the beginning and the end.
We keep whitespace for a certain indentation level, but additional
whitespace gets preserved. This is less important, because regular
spaces is indeed irrelevant. But when we write the annotations, we
should be in full control over spaces.

With the given input, this produces *exactly* the same XML as before.

- the parsing is now stricter (and thus the code more verbose). No
  funny stuff, get the annotations correct.
- on parsing errors, we log now the affecting lines
- "nm-setting-ip-config.c" is a base class. Previously it was ignored
  and for the moment we still do that. Next, we will allow to also
  describe properties there.
- prepare the code to better preserve whitespace, indentation and line
  wrappings. In particular, to honor a blank line to indicate a line break
  and support paragraphs. This is not yet done to compare the output
  to before, but will be turned on with a small patch next.
- the code will make it simple to promote the XML attributes to
  nodes. Attributes aren't great, let's write XML nodes later.
  We will only need to adjust the "keywords" dictionary for that,
  but this change will require changes to the entire chain of tools.

From nm-settings-dbus(5):

Before (ugly, offensive, possibly in violation of geneva protocol):

  ...
  | set, the authentication retries for 3
  | times before failing the connection.
  |
  |  Currently, this only applies to 802-1x
  | authentication.

After (beautiful, smells good, in harmony with nature):

  ...
  | set, the authentication retries for 3
  | times before failing the connection.
  |
  | Currently, this only applies to 802-1x
  | authentication.