Use open_normal_and_derived_tables instead of open_and_lock_tables when reading metadata for a table.
 Add two test cases, one for "USE database" and one for "SHOW COLUMNS FROM table"


mysql-test/r/lock_multi.result:
  Updated test results for test case for bug9998
mysql-test/r/mysql.result:
  Updated results for test case for bug9998
mysql-test/t/lock_multi.test:
  Test that "show columsn from t1" is not locked by another thread having a write lock on t1
mysql-test/t/mysql.test:
  Add test case for BUG9998
    - lock table t1 for write from mysql-test, then execute 'mysql' and call "USE test". This will test exactly what caused the bug.
sql/sql_show.cc:
  Open tables without locking when reading metadata