Support managing the loopback interface through NM as the users want to
set the proper mtu for loopback interface when forwarding the packets.
Additionally, the IP addresses, DNS, route and routing rules are also
allowed to configure for the loopback connection profiles.

https://bugzilla.redhat.com/show_bug.cgi?id=2060905

Turns out automake doesn't like either that we add dependencies to a
target conditionally. Not sure why, but it's a tasteless thing to do
nevertheless:

  gtk-doc.make:174: warning: $(DOC_MAIN_SGML_FILE) was already defined in condition BUILD_DOCS, which is included in condition TRUE ...
  docs/api/Makefile.am:112:   'gtk-doc.make' included from here
  docs/api/Makefile.am:14: ... '$(DOC_MAIN_SGML_FILE)' previously defined here

In any case, the dependency is wrong in the first place -- the source
document doesn't depend on other source. The target (which is a stamp
file, since the xslt processor produces multiple files) does! Moreover,
there's a dependency of the stamp file on $(content_files) already, so
including GENERATED_FILES (like we already do) in content_files should
be sufficient.

While at that, deal with other not-so-nice things; don't extend "all"
target needlessly and don't define a target conditionally.

NetworkManager-wait-online is a constant source of confusion,
as it seems to delay the boot (when it's often just the messenger
or either a network problem, a NetworkManager misconfiguration
or a misconfiguration of other systemd services).

Try to clear that up with a manual page.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1130

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/784

"libnm-core/" is rather complicated. It provides a static library that
is linked into libnm.so and NetworkManager. It also contains public
headers (like "nm-setting.h") which are part of public libnm API.

Then we have helper libraries ("libnm-core/nm-libnm-core-*/") which
only rely on public API of libnm-core, but are themself static
libraries that can be used by anybody who uses libnm-core. And
"libnm-core/nm-libnm-core-intern" is used by libnm-core itself.

Move "libnm-core/" to "src/". But also split it in different
directories so that they have a clearer purpose.

The goal is to have a flat directory hierarchy. The "src/libnm-core*/"
directories correspond to the different modules (static libraries and set
of headers that we have). We have different kinds of such modules because
of how we combine various code together. The directory layout now reflects
this.

These SPDX license identifiers are deprecated ([1]). Update them.

[1] https://spdx.org/licenses/

  sed \
     -e '1 s%^/\* SPDX-License-Identifier: \(GPL-2.0\|LGPL-2.1\)+ \*/$%/* SPDX-License-Identifier: \1-or-later */%' \
     -e '1,2 s%^\(--\|#\|//\) SPDX-License-Identifier: \(GPL-2.0\|LGPL-2.1\)+$%\1 SPDX-License-Identifier: \2-or-later%' \
     -i \
     $(git grep -l SPDX-License-Identifier -- \
         ':(exclude)shared/c-*/' \
         ':(exclude)shared/n-*/' \
         ':(exclude)shared/systemd/src' \
         ':(exclude)src/systemd/src')

https://bugzilla.redhat.com/show_bug.cgi?id=1867997
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/ ## 600

I still don't understand why we get now these ".actions" build
artifacts. Anyway, I don't think we need to care. Just ignore
it.

"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.