updates

Attempt to execute trigger or stored function with multi-UPDATE
which used - but didn't update - a table that was also used by
the calling statement led to an error. Read-only reference to
tables used in the calling statement should be allowed.
 
This problem was caused by the fact that check for conflicting
use of tables in SP/triggers was performed in open_tables(),
and in case of multi-UPDATE we didn't know exact lock type at
this stage.

We solve the problem by moving this check to lock_tables(), so
it can be performed after exact lock types for tables used by
multi-UPDATE are determined.


mysql-test/r/trigger.result:
  Results for the added test case is added.
mysql-test/t/trigger.test:
  A new test case is added, verifying correct table multi-update
  conflict resolution, both read-only and write.
sql/sql_base.cc:
  The check for conflicting use of tables in SP/triggers is moved
  to lock_tables(), to be performed after the exact lock types
  have been determined. Also, an assert is added to open_ltable()
  to ensure this func is not used in a prelocked context.