Added command 'replace_column' to mysqltest


client/mysqltest.c:
  Added user command replace_column
mysql-test/r/merge.result:
  Update of tests
mysql-test/t/innodb.test:
  User replace_column to make tests repeatable
mysql-test/t/merge.test:
  New tests for testing MERGE tables with multiple databases
scripts/mysql_create_system_tables.sh:
  Fixed syntax error
sql/ha_myisammrg.cc:
  Allow one to use MERGE tables with tables from different databases
sql/mysql_priv.h:
  Added function
sql/sql_parse.cc:
  Allow one to use MERGE tables with tables from different databases
sql/sql_show.cc:
  Extended append_identifier()