Problem: crash on attempt to open a table
  having "#mysql50#" prefix in db or table name.
  Fix: This prefix is reserved for "mysql_upgrade"
  to access 5.0 tables  whose file names are not encoded
  according to "5.1 tablename to filename encoded".
  Don't try open tables whose db name or table name
  has this prefix.


mysql-test/r/show_check.result:
  Adding test case.
mysql-test/t/show_check.test:
  Adding test case.
sql/mysql_priv.h:
  Moving 5.0 prefix declarations into mysql_priv.h
sql/sql_table.cc:
  Moving 5.0 prefix declarations into mysql_priv.h
sql/table.cc:
  Don't try to do "normal" open of tables having '#mysql50#' prefix in
  db or table name. This prefix is reserved to access to unencoded
  table names when upgrading from 5.0 to 5.1.