tables' lock."

Execution of ALTER TABLE ... ENABLE KEYS on a table (which can take rather
long time) prevented concurrent execution of all statements using tables.

The problem was caused by the fact that we were holding LOCK_open mutex
during whole duration of this statement and particularly during call
to handler::enable_indexes(). This behavior was introduced as part of the
fix for bug 14262 "SP: DROP PROCEDURE|VIEW (maybe more) write to binlog
too late (race cond)"

The patch simply restores old behavior. Note that we can safely do this as
this operation takes exclusive lock (similar to name-lock) which blocks both
DML and DDL on the table being altered.

It also introduces mysql-test/include/wait_show_pattern.inc helper script
which is used to make test-case for this bug robust enough.