Convenient for copy & paste.

From Fedora 28 on we can build without Python 2. That is good,
because it's eventually going to be removed.

Based on a change in Fedora dist-git by Iryna Shcherbina.

https://bugzilla.redhat.com/show_bug.cgi?id=1581693

This makes package updates more robust, avoiding in-place replaces of
the plugins.

Previously, if an upgrade transaction was terminated, NetworkManager
library could end up being of a different version than the plugins.
If the user was unfortunate enough to connect using a connection that
required a plugin (say, Wi-Fi), he would be left without a network
connection making it somewhat inconvenient to recover from the botched
upgrade.

This makes the whole situation a little bit less sad.

The VPN plugins are kept where they always have been -- the path is not
qualified with a version number.

When built with iwd support, add an option to use iwd in place of
wpa_supplicant.

The "boolean dependencies" are only supported since RPM 4.13, with older
versions just keep things the way they were before.

This is broken -- looks for VPN plugins in a version-qualified path,
which is wrong and break installed VPN plugins.

This reverts commit 6384ad4ee16f31dd103f790bd193c4049d89a6fc.

This makes package updates more robust, avoiding in-place replaces of
the plugins.

Previously, if an upgrade transaction was terminated, NetworkManager
library could end up being of a different version than the plugins.
If the user was unfortunate enough to connect using a connection that
required a plugin (say, Wi-Fi), he would be left without a network
connection making it somewhat inconvenient to recover from the botched
upgrade.

This makes the whole situation a little bit less sad.

On RHEL, we don't have NetworkManager-config-connectivity-fedora package.
Hence, the spec files for RHEL differ from upstream in this regard.

The aim is that contrib/rpm's spec file can be used almost as-is for
RHEL, Fedora and possibly other distros. Hence, build the subpackage
conditionally to minimize the difference.

They were not (notably) touched in more than 3 years.
I doubt anybody is using them.

Also, nowadays we have contrib/rpm to build NetworkManager
packages for Fedora/RPM. We have copr, we have automated CI
in CentOS CI and beaker.

Also, nowadays it should be easy to spawn a a fedora image
in a container or tools like vagrant.

I think there are better alternatives. Drop the scripts.

From libnl3, we only used the helper function to parse/generate netlink
messages and the socket functions to send/receive messages. We don't
need an external dependency to do that, it is simple enough.

Drop the libnl3 dependency, and replace all missing code by directly
copying it from libnl3 sources. At this point, I mostly tried to
import the required bits to make it working with few modifications.

Note that this increases the binary size of NetworkManager by 4736 bytes
for contrib/rpm build on x86_64. In the future, we can simplify the code
further.

A few modifications from libnl3 are:

- netlink errors NLE_* are now in the domain or regular errno.
  The distinction of having to bother with two kinds of error
  number domains was annoying.

- parts of the callback handling is copied partially and unused parts
  are dropped. Especially, the verbose/debug handlers are not used.
  In following commits, the callback handling will be significantly
  simplified.

- the complex handling of seleting ports was simplified. We now always
  let kernel choose the right port automatically.