contrib: improve motd for nm-in-container.sh script

Leave a hint about core-dumps.

Also, now we have `contrib/fedora/rpm/configure-for-system.sh` script,
which can configure the build in a way similar to what we get
when doing an RPM build.

That means, inside "contrib/scripts/nm-in-container.sh" we
can just type `make install`, and it will replace the pre-installed
NetworkManager.

The main advantage is that it becomes convenient to run NetworkManager
as a systemd service. Previously, the suggested was to to install
NetworkManager inside another prefix, and run it in the terminal.

Running NetworkManager as systemd service is also necessary for NM-ci,
which restarts the NetworkManager service, and you couldn't run a test,
if you just started NetworkManager in a terminal.

Previously, you had to build a complete RPM, which takes a lot of time.
61 jobs for th/in-container-inplace in 1 minute and 58 seconds (queued for 12 seconds)