using GROUP BY"
Now we are setting Field_timestamp::field_length to 19 in open_table()
if we are in new mode (and we are restoring it back when we are coming
back to normal mode). This also should solve potential problems with
some of LOAD DATA INFILE and SELECT * INTO in this mode.


mysql-test/r/type_timestamp.result:
  Added test for bug #4131 'TIMESTAMP columns missing minutes and seconds
  when using GROUP BY' and other --new mode related behavior.
mysql-test/t/type_timestamp.test:
  Added test for bug #4131 'TIMESTAMP columns missing minutes and seconds
  when using GROUP BY' and other --new mode related behavior.
sql/field.cc:
  Added Field_timestamp::orig_field_length member for saving original
  field_length value, because this member can be modified if new_mode is
  in effect.
  Lot of Field_timestamp code simplified and Field_timestamp::make_field()
  is no longer needed because we are setting field_length to 19 if we are
  in --new mode now.
sql/field.h:
  Added Field_timestamp::orig_field_length member for saving original
  field_length value, because this member can be modified if new_mode
  is in effect. 
  Field_timestamp::make_field() is no longer needed because we are setting
  field_length to 19 if we are in --new mode now.
sql/sql_base.cc:
  If --new mode is in effect all TIMESTAMP fields should pretend that they
  have length of 19. We are achieving this by setting 
  Field_timestamp::field_length to 19 (or original value) in open_table().
  We are using TABLE::timestamp_mode variable for avoiding of unnecessary
  looping over all fields of the table and setting field_length if table
  was used with same new_mode value before.
  
  Note: We do not introduce general framework for setting up Field objects
  for usage with current THD here because this fix is only needed in 4.0
  and Monty said that we will also remove looping over all fields when
  updating table_name member at some point. This more general framework
  will also complicate nice optimization with avoiding of unneeded looping.
sql/sql_parse.cc:
  Now when we are creating TIMESTAMP(19) fields by default in --new mode,
  otherwise we will have unaligned behavior between ALTER and CREATE.
sql/table.h:
  Added TABLE::timestamp_mode field for saving information whenever we set 
  field_length members of table's TIMESTAMP fields to 19 (to honor 
  new_mode) or they have original values.