Fixes ModemManager#638

Just be patient if you're waiting for a merge request review.

https://sigquit.wordpress.com/2021/11/18/fcc-unlock-procedure-updates-in-modemmanager-1-18-4/#comment-27523

Make it equivalent to the command used for all files. Also suggested
by heftig https://sigquit.wordpress.com/2021/11/18/fcc-unlock-procedure-updates-in-modemmanager-1-18-4/#comment-27376

As suggested by heftig here:
https://sigquit.wordpress.com/2021/11/18/fcc-unlock-procedure-updates-in-modemmanager-1-18-4/#comment-27372

Renamed the directory where the example scripts are installed, and
modified the steps to manually install them so that the user only
needs to create symlinks under ${sysconfdir}.

Also made reference to the path that third party packages should use
to install their FCC unlock tools, under ${libdir}.

Hopefully.

We need --sysconfdir since 1.18.4 so that the runtime lookup path for
FCC unlock scripts is /etc/ModemManager/fcc-unlock.d, instead of
/usr/etc/ModemManager/fcc-unlock.d.