column types TIMESTAMP is NOT NULL by default, so in order to have 
TIMESTAMP column holding NULL valaues you have to specify NULL as
one of its attributes (this needed for backward compatibility).

Main changes:
Replaced TABLE::timestamp_default_now/on_update_now members with
TABLE::timestamp_auto_set_type flag which is used everywhere
for determining if we should auto-set value of TIMESTAMP field 
during this operation or not. We are also use Field_timestamp::set_time()
instead of handler::update_timestamp() in handlers.


mysql-test/r/type_timestamp.result:
  Added test for TIMESTAMP columns which are able to store NULL values.
mysql-test/t/type_timestamp.test:
  Added test for TIMESTAMP columns which are able to store NULL values.
sql/field.cc:
  Added support for TIMESTAMP fields holding NULL values.
  We don't need Field_timestamp::set_timestamp_offsets() anymore.
  Instead we need Field_timestamp::get_auto_set_type() function
  which will convert TIMESTAMP auto-set type stored in Field in 
  unireg_check to value from timestamp_auto_set_type_enum.
  (We can't replace this function with additional Field_timestamp member
  and some code in constructor because then we will have troubles
  with Field::new_field() method).
  We should also set field to not null in Field_timestamp::set_time() now.
sql/field.h:
  Added support for TIMESTAMP fields holding NULL values.
  We don't need Field_timestamp::set_timestamp_offsets() anymore.
  Instead we need Field_timestamp::get_auto_set_type() function,
  which will convert TIMESTAMP auto-set type stored in Field in 
  unireg_check to value from timestamp_auto_set_type_enum.
  We also have to support NULL values in Field_timestamp::get_timestamp()
  function.
sql/field_conv.cc:
  Added comment clarifying behavior in case of TIMESTAMP fields which are
  able to store NULL values.
sql/ha_berkeley.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now for determining 
  if we should auto-set value of TIMESTAMP field during this operation.
  We are also use Field_timestamp::set_time() instead of 
  handler::update_timestamp().
sql/ha_heap.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now for determining 
  if we should auto-set value of TIMESTAMP field during this operation.
  We are also use Field_timestamp::set_time() instead of 
  handler::update_timestamp().
sql/ha_innodb.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now for determining 
  if we should auto-set value of TIMESTAMP field during this operation.
  We are also use Field_timestamp::set_time() instead of 
  handler::update_timestamp().
sql/ha_isam.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now for determining 
  if we should auto-set value of TIMESTAMP field during this operation.
  We are also use Field_timestamp::set_time() instead of 
  handler::update_timestamp().
sql/ha_isammrg.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now for determining 
  if we should auto-set value of TIMESTAMP field during this operation.
  We are also use Field_timestamp::set_time() instead of 
  handler::update_timestamp().
sql/ha_myisam.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now for determining 
  if we should auto-set value of TIMESTAMP field during this operation.
  We are also use Field_timestamp::set_time() instead of 
  handler::update_timestamp().
sql/ha_myisammrg.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now for determining 
  if we should auto-set value of TIMESTAMP field during this operation.
  We are also use Field_timestamp::set_time() instead of 
  handler::update_timestamp().
sql/ha_ndbcluster.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now for determining 
  if we should auto-set value of TIMESTAMP field during this operation.
  We are also use Field_timestamp::set_time() instead of 
  handler::update_timestamp().
sql/handler.cc:
  handler::update_timestamp() is no longer needed since now we use
  Field_timestamp::set_time() instead.
  (we can't use handler::update_timestamp() anyway since field position
  only is not enough for TIMESTAMP fields which are able to store NULLs)
sql/handler.h:
  handler::update_timestamp() is no longer needed since now we use
  Field_timestamp::set_time() instead.
sql/item_timefunc.cc:
  Since now TIMESTAMP fields can hold NULL values we should take this into
  account.
sql/sql_base.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now. 
  (Here we use Field_timestamp::get_auto_set_type() to setup its value
   before further statement execution).
sql/sql_insert.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now.
sql/sql_load.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now.
sql/sql_parse.cc:
  Added support for TIMESTAMP fields holding NULL values.
  We should distinguish NULL default values and non-specified default
  values for such fields (because latter could mean DEFAULT NOW()
  ON UPDATE NOW() in some cases).
sql/sql_show.cc:
  Added support for TIMESTAMP fields holding NULL values.
  Unlike all other fields these are NOT NULL by default
  so we have to specify NULL attribute explicitly for them.
sql/sql_table.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now.
sql/sql_update.cc:
  Now we use TABLE::timestamp_field_type instead of 
  TABLE::timestamp_default_now/on_update_now.
sql/sql_yacc.yy:
  Added support for TIMESTAMP fields holding NULL values.
  Unlike all other fields these are NOT NULL by default
  (so we have to set NOT_NULL_FLAG properly for them).
sql/table.h:
  Added timestamp_auto_set_type enum which values are used for indicating
  during which operations we should automatically set TIMESTAPM field
  value to current timestamp.
  TABLE: Replaced timestamp_default_now/on_update_now members with
  timestamp_auto_set_type flag (Now when TIMESTAMP field are able to 
  store NULL values, single position of field in record is not enough 
  for updating this field anyway).