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.

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.