WL#909: New sql mode 'dump' to make reliable backups of timestamps

Currently we have a problem that if you do a mysqldump + import, you may loose 
timestamp information for hours during daylight saving time changes because 
timestamps are exported / imported as a string.

The solution for this is to do the following extension to timestamp handling:
When inserting a binary hex value (like 0x3EDFEA8A) we should insert the binary
value exacly like it is into a timestamp field.

(Above hex string is taken from: hex(unix_timestamp(now())) )

This should be fixed in the Item_varbinary::save_in_field() which should 
check if the recieving field is of type 'timestamp' and if it is, store the
value unconverted in the field.

We should also extend mysqldump to set sql_mode to 'dump' when doing backups.

This is to be done in the 4.1 tree.