The --initialize* mode was not initializing the ACL cache and thus
all CREATE|DROP|ALTER USER commands and all GRANT|REVOKE commands
were failing.
Since --init-file commands were executed in the same mode all of
the above ACL related statements were failing in them too. This was
unline the "normal" server startup where, when the --init-file is
played, the ACL cache is already initialized.

Initialized the ACL cache (through invoking FLUSH PRIVILEGES) as
part of the --initialize sequence right after the mysql_system_tables.sql
replay and before the creation of the root@localhost user and subsequent
commands.

Fixed the --initialize code to use CREATE USER and GRANT statements
instead of the INSERT statements it was using.

Relaxed a condition of not running as an anonymous user when changing
passwords, since the --initialize thread is running as one such user.

Test case added.