To-number conversion warnings work differenly with CHAR 
and VARCHAR sp variables.

The original revision-IDs are:
  staale.smedseng@sun.com-20081124095339-2qdvzkp0rn1ljs30
  staale.smedseng@sun.com-20081125104611-rtxic5d12e83ag2o
                                                
The patch provides ER_TRUNCATED_WRONG_VALUE warning messages
for conversion of VARCHAR to numberic values, in line with
messages provided for CHAR conversions. Conversions are
checked for success, and the message is emitted in case
failure.
                                                
The tests are amended to accept the added warning messages,
and explicit conversion of ON/OFF values is added for
statements checking system variables. In test
rpl.rpl_switch_stm_row_mixed checking for warnings is
temporarily disabled for one statement, as this generates
warning messages for strings that vary between executions.


sql/field.cc:
  The pushing of the truncation warning is now done in a
  separate static function, and used in various places.