(backport)
   mysql_upgrade script accepts --upgrade-system-tables option,
   fixing only system tables in this case.

per-file comments:
  client/mysql_upgrade.c
WL#4991 mysql_upgrade --fix-privilege-tables
    --upgrade-system-tables option added.
   if it is set, the tool won't look for the mysqlcheck then
   run_mysqlcheck_fixnames() and run_mysqlcheck_upgrade won't be called.
  mysql-test/r/mysql_upgrade.result
WL#4991 mysql_upgrade --fix-privilege-tables
    test result added
  mysql-test/t/mysql_upgrade.test
WL#4991 mysql_upgrade --fix-privilege-tables
    test case added