data directory name command

The check_db_name function has been modified to validate tails of
#mysql50#-prefixed database names for compliance with MySQL 5.0
database name encoding rules (the check_table_name function call
has been reused).


mysql-test/r/renamedb.result:
  Updated test case.
mysql-test/r/upgrade.result:
  Test case for bug #53804.
mysql-test/t/renamedb.test:
  Updated test case.
mysql-test/t/upgrade.test:
  Test case for bug #53804.
sql/mysql_priv.h:
  Bug #53804: serious flaws in the alter database .. upgrade
              data directory name command
  
  The check_mysql50_prefix has been added.
sql/sql_table.cc:
  Bug #53804: serious flaws in the alter database .. upgrade
              data directory name command
  
  - The check_mysql50_prefix has been added.
  - The check_n_cut_mysql50_prefix function has been refactored
  to share code with new check_mysql50_prefix function.
sql/table.cc:
  Bug #53804: serious flaws in the alter database .. upgrade
              data directory name command
  
  The check_db_name function has been modified to validate tails of
  #mysql50#-prefixed database names for compliance with MySQL 5.0
  database name encoding rules.