This option is already present in the wireguard module, but missing from
the wg-quick module. This is very annoying, because it means you can't
easily get a safe and valid configuration on first boot when using
wg-quick.
This change adds the same option with the same description text and the
same script, but instead of generating an entire systemd unit dedicated
to creating the key file, it adds the script as a PreUp script, which
is a much simpler solution.
I've tested this in my own configuration, and it does indeed work.
wg-quick allows multiple PreUp scripts, which are run in order, and
all PreUp scripts are run before the private key is read from disk,
see `man wg-quick`.
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