pam v2: mysql_upgrade doesn't fix the ownership/privileges of auth_pam_tool

As part of MDEV-19876, some code was added to mysql_install_db that fixes the ownership and privileges of auth_pam_tool:

https://github.com/mariadb/server/commit/11f3e2366282eb8cf1cb0062793d102067db6472

Now, we have code that fixes the ownership and privileges of auth_pam_tool in the following cases:

• If the MariaDB-server RPM is installed by the root user, then it will properly set the ownership and privileges of the auth_pam_tool binary.

• If the mariadb-server DEB is installed by the root user, then it will properly set the ownership and privileges of the auth_pam_tool binary.

• If mysql_install_db is run by the root user, then it will properly set the ownership and privileges of the auth_pam_tool binary.

This still leaves a big gap, because:

• If someone is using a binary tarball installation, then the RPM and DEB improvements won't help them.
• If someone is performing an upgrade, rather than a fresh install, then the mysql_install_db improvements won't help them.

It seems as though mysql_upgrade should also try to fix the ownership and privileges of auth_pam_tool, so that these cases are also covered.