Store the name of the hash function along with the hashed password (refs: #17135)

... so that we can detect when the hashed password becomes incompatible
with the PAM config (in case that the hash function used by PAM
changes).

config/chroot_local-includes/usr/lib/python3/dist-packages/tailsgreeter/settings/admin.py

@@ -26,6 +26,7 @@ class AdminSetting(object):
 
         write_settings(self.settings_file, {
             'TAILS_USER_PASSWORD': pipes.quote(hashed_and_salted_pw),
+            'TAILS_PASSWORD_HASH_FUNCTION': 'SHA512',
         })
         logging.debug('password written to %s', self.settings_file)