Docs/manual.texi:
  ChangeLog
sql/field.h:
  New virtual function to set a field to null or signal an error
sql/field_conv.cc:
  New function to set a field to null or signal an error
sql/item.cc:
  When setting a field to null internally (for WHERE testing) don't autoconvert NULL -> now() or last_insert_id()
sql/item.h:
  New virtual function to set a field to null or signal an error