WL#4991: mysql_upgrade --fix-privilege-tables

Have an option to mysql_upgrade to only execute the SQL that upgrades the system
tables:
--fix-privileges-tables.

mysql_upgrade is an all-in-one program that a user must run after upgrading to
the next GA release of MySQL.

MySQL upgrade will:
 - create new system tables in mysql database
 - update definitions of existing mysql. system tables
 - check the names of the databases on the file system, to make sure that they will
work properly with tablename-to-filename encoding
 - etc.

Prior to MySQL 5.4, in addition to mysql_upgrade the server would distribute two
SQL scripts:
mysql_system_tables.sql and mysql_system_tables_fix.sql

Prior to that, mysql_system_tables_fix.sql was called mysql_fix_privilege_tables.sql

Before introduction of mysql_upgrade program, the only "fix" on upgrade that was
needed
was to update the privilege tables -- later to become "system" tables.

Starting with 5.4 we no longer include the SQL scripts mentioned above into the
server installation, in other words, do not allow the user to run them directly.
Still to allow for some troubleshooting, we introduce an option to mysql_upgrade
to only
upgrade the system tables, in other words, not attempt to upgrade the data.

The name of the option will be:

-upgrade-system-tables

(--fix-privilege-tables is a legacy name of an old script that we do not want to
reincarnate in the option name).

Something as simple as adding a new option and modifying this piece of main():
  if (run_mysqlcheck_fixnames() ||
      run_mysqlcheck_upgrade() ||
      run_sql_fix_privilege_tables())