Bug #18361: Triggers on mysql.user table cause server crash

 Because they do not work, we do not allow creating triggers on tables
 within the 'mysql' schema.

 (They may be made to work and re-enabled at some later date, but not
 in 5.0 or 5.1.)


mysql-test/r/trigger.result:
  Add new results
mysql-test/t/trigger.test:
  Add new regression test for creating triggers on system schema
sql/share/errmsg.txt:
  Add new error message
sql/sql_trigger.cc:
  Disallow creating triggers on tables in the 'mysql' schema