command and reported to a client.

The fact that a timestamp field will be set to NO on UPDATE wasn't shown 
by the SHOW COMMAND and reported to a client through connectors. This led to
problems in the ODBC connector and might lead to a user confusion.

A new filed flag called ON_UPDATE_NOW_FLAG is added. 
Constructors of the Field_timestamp set it when a field should be set to NOW
on UPDATE.

The get_schema_column_record function now reports whether a timestamp field
will be set to NOW on UPDATE.