contrib/rpm: split ifup/ifdown compat scripts to new package NetworkManager-initscripts-updown

Previously (on RHEL<=8 and Fedora<=35), NetworkManager package contains
the compat scripts nm-ifup/nm-ifdown.

If initscripts package (not network-scripts!) is installed, then a RPM
trigger links them as alternatives for the ifup/ifdown commands.

One problem is that `dnf provides /usr/sbin/ifup` lists the
NetworkManager package. Which is technically true, but on RHEL9 where
initscripts is not installed by default, `dnf install NetworkManager`
does not actually create those scripts.

Solve that by moving those scripts to a new subpackage
NetworkManager-initscripts-updown. The %post script now always creates the
alternatives links, regardless whether initscripts package is installed.

Note that on RHEL8, NetworkManager package not only Obsoletes: but also
Suggests: the new package.

The name "initscripts-updown" is chosen because in the future we might
have additonal initscripts/ifcfg related subpackages to contain the
ifcfg-rh plugin (NetworkManager-initscripts-ifcfg) or ifcfg-rh migration
tools (NetworkManager-initscripts-tools).
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