Do not consider SHOW commands slow queries, just because they don't use proper indexes.

This bug fix is not needed in 5.1, and the code changes will be null merged.  However, the test cases will be propogated up to 5.1.


mysql-test/t/show_check-master.opt:
  Rename: BitKeeper/deleted/.del-show_check-master.opt -> mysql-test/t/show_check-master.opt
mysql-test/r/ps.result:
  Add test case for bug 19764
mysql-test/r/show_check.result:
  Add test case for bug 19764
mysql-test/r/union.result:
  Adjust test case results based on bug #19764 changes
mysql-test/t/ps.test:
  Add test case for bug 19764
mysql-test/t/show_check.test:
  Add test case for bug 19764
sql/sql_parse.cc:
  Do not log SHOW commands as slow queries just because they don't use indexes.
sql/sql_prepare.cc:
  Save stmt_backup.lex->orig_sql_command for use in log_slow_statement()
mysql-test/t/ps-master.opt:
  Add log-slow-queries and --log-queries-not-using indexes, to test bug #19764