On Fedora 33 when compiling NetworkManager it is failing with the
following error:

```
ERROR: files left in build directory after distclean:
./docs/libnm/libnm.actions
make[1]: *** [Makefile:18427: distcleancheck] Error 1
make[1]: Leaving directory '/builddir/nm-build/NetworkManager/NetworkManager-1.27.90/_build/sub'
make: *** [Makefile:18356: distcheck] Error 1
Error make distcheck
```

Adding the file to the DISTCLEANFILES will enforce the removal of this
file.

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/659

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/565

"nm-settings-docs-nmcli.xml" will be generated by a tool that depends on
"clients/common/". The file should thus not be in libnm directory, otherwise
there is a circular dependency.

Move the file to "man/" directory.

For consistency, also move "nm-settings-docs-dbus.xml". Note that we
cannot move "nm-settings-docs-gir.xml" to "man/", because that one is
needed for building clients.

Like the previous commit. Move code that depends on libnm out
of shared to avoid circular dependency.

Also add a readme file explaining the reason for existence of
the helper library.

Like the previous commit. Move code that depends on libnm-core out
of shared to avoid circular dependency.

Also add a readme file explaining the reason for existence of
the helper libraries nm-libnm-core-intern and nm-libnm-core-aux.

The "shared" directory is used by libnm-core, it should thus only depend on
code that is in the "shared" directory. Otherwise there is a circular
dependency, and meson's subdir() does not work nicely.

Also, libnm-core is really part of (and also an extension of) libnm-core,
so it belongs there.

I guess, the original idea was that this is also an extension for libnm,
so another project could take these utility functions (by copying them
into their source tree) and use them. That is still possible, it's
just that the sources are no longer under the shared directory.

Also add a readme to explain the non-obvious meaning of these files.

Originally, these files were part of libnm-core and linked together.
However, that is a licensing violation, because the code is GPL-2.0+
licensed, while libnm-core also gets linked with libnm (it must thus
be LGPL-2.1+). The original intent behind moving the code to "shared/"
was to avoid the licensing issue, but also to prepare when we would add
a separate, GPL licensed libnm-keyfile. However, currently we hope to
be able to relicense the code, so that it actually could be exposed as
part of libnm. This is work in progress at ([1]).

[1] https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/ ## 517

Anyway, the current directory layout is problematic. libnm-keyfile
depends on libnm-core, while libnm-core depends on code under shared.
That means, there is a circular dependency and meson's subdir() does
not work well.

Move the code.

A significant part of NetworkManager's API are the connection profiles, documented
in `man nm-settings*`. But there are different aspects about profiles, depending
on what you are interested. There is the D-Bus API, nmcli options, keyfile format,
and ifcfg-rh format. Additionally, there is also libnm API.

Add distinct manual pages for the four aspects. Currently the two new manual
pages "nm-settings-dbus" and "nm-settings-nmcli" are still identical to the
former "nm-settings.5" manual. In the future, they will diverge to
account for the differences.

There are the following aspects:

 - "dbus"
 - "keyfile"
 - "ifcfg-rh"
 - "nmcli"

For "libnm" we don't generate a separate "nm-settings-libnm" manual
page. That is instead documented via gtk-doc.

Currently the keyfile and ifcfg-rh manual pages only detail settings
which differ. But later I think also these manual pages should contain
all settings that apply.

"nm-settings-docs-dbus.xml" is "nm-settings-docs-gir.xml" merged with
"nm-property-infos-dbus.xml". The name should reflect that, also because
we will get more files with this naming scheme.

"man/nm-settings%.xml" really should depend on "common.ent".
The reason is that XSL files like "man/nm-settings.xsl" include
"common.ent".
The previous code already tried to express that, but for some
reasons this dependency was not honored. Fix that.

However, that uncovers another problem with gtk-doc.make. If we do
that without the workaround for "docs/api/Makefile.am", then

  $ ./autogen.sh && make V=1 SHELL='sh -x' distcheck

breaks.

The reason is not clear to me. The new dependency leads to rebuild
"man/nm-settings-keyfile.xml". But that is worse, somehow the file
"$(top_srcdir)/man/nm-settings-keyfile.xml" ends up being read-only.
Afterwards, gtk-doc.make does

    setup-build.stamp:
        -$(GTK_DOC_V_SETUP)if test "$(abs_srcdir)" != "$(abs_builddir)" ;
        then \
          files=`echo $(SETUP_FILES) $(DOC_MODULE).types`; \
          if test "x$$files" != "x" ; then \
            for file in $$files ; do \
              destdir=`dirname $(abs_builddir)/$$file`; \
              test -d "$$destdir" || mkdir -p "$$destdir"; \
              test -f $(abs_srcdir)/$$file && \
                cp -pf $(abs_srcdir)/$$file $(abs_builddir)/$$file || true;
                \
            done; \
          fi; \
        fi
        $(AM_V_at)touch setup-build.stamp

so that the files in build dir are also read-only. Then, make distcheck
goes ahead and builds the files once again, which fails.

You are welcome to understand why this workaround is necessary. Please
then create a better fix.