By calling chpasswd with the -e option, it uses the provided hashed and
salted password instead of hashing and salting it via PAM.

PAM uses SHA512 to hash the password, as configured in /etc/login.defs,
so in the call to mkpasswd we set --method=sha512crypt to also use SHA512.

To make it easier to persist all of these settings.

Apply greeter-buster-blocking-tails-unblock-network.diff to the Greeter files now shipped here (refs: #16912)

Those are the files installed by the greeter, from the master branch of
the greeter repo (commit 2b1facdbe3c4c9f209cdcf3835a9d52a3147b8a8).

All files are copied to the same path they are installed at when
installing the greeter Debian package, except for:

 * The tailsgreeter Python package, which is now installed under
   /usr/local/lib/python3/ instead of /usr/lib/python3, because
   that's where we put our own Python packages.

 * The tails-greeter script, which is now installed in /usr/local/lib/
   instead of /usr/bin, because this script should not be executed by
   the user directly, similar to the other scripts in the lib directory.
   The Exec= path in tails-greeter.desktop is updated accordingly.