If we are in stored function or trigger we should ensure that we won't change
table that is already used by calling statement (this can damage table or
easily cause infinite loops). Particularly this means that recursive triggers
should be disallowed.


mysql-test/r/sp-error.result:
  Added tests checking that in functions we don't allow to update tables which
  are used by statements which invoke these functions.
mysql-test/r/trigger.result:
  Added test for bug #11896 "Partial locking in case of recursive trigger
  definitions".
mysql-test/t/sp-error.test:
  Added tests checking that in functions we don't allow to update tables which
  are used by statements which invoke these functions.
mysql-test/t/trigger.test:
  Added test for bug #11896 "Partial locking in case of recursive trigger
  definitions".
sql/share/errmsg.txt:
  Added error messages for complaining about situations when in function or
  trigger we try to change table which is used in statement invoking this
  function or trigger.
sql/sql_base.cc:
  open_table():
    If we are in stored function or trigger we should ensure that
    we won't change table that is already used by calling statement
    (this can damage table or easily cause infinite loops).
    So if we are opening table for writing, we should check that it
    is not already open by some calling stamement.