mode.

When a new DATE/DATETIME field without default value is being added by the
ALTER TABLE the '0000-00-00' value is used as the default one. But it wasn't
checked whether such value was allowed by the set sql mode. Due to this
'0000-00-00' values was allowed for DATE/DATETIME fields even in the
NO_ZERO_DATE mode.

Now the mysql_alter_table() function checks whether the '0000-00-00' value
is allowed for DATE/DATETIME fields by the set sql mode.
The new error_if_not_empty flag is used in the mysql_alter_table() function
to indicate that it should abort if the table being altered isn't empty.
The new new_datetime_field field is used in the mysql_alter_table() function
for error throwing purposes. 
The new error_if_not_empty parameter is added to the copy_data_between_tables()
function to indicate the it should return error if the source table isn't empty.


mysql-test/t/alter_table.test:
  Added a test case for the bug#27507: Wrong DATETIME value was allowed by
  ALTER TABLE in the NO_ZERO_DATE mode.
mysql-test/r/alter_table.result:
  Added a test case for the bug#27507: Wrong DATETIME value was allowed by
  ALTER TABLE in the NO_ZERO_DATE mode.
sql/sql_table.cc:
  Bug#27507: Wrong DATETIME value was allowed by ALTER TABLE in the NO_ZERO_DATE
  mode.
  Now the mysql_alter_table() function checks whether the '0000-00-00' value
  is allowed for DATE/DATETIME fields by the set sql mode.
  The new error_if_not_empty flag is used in the mysql_alter_table() function
  to indicate that it should abort if the table being altered isn't empty.
  The new new_datetime_field field is used in the mysql_alter_table() function
  for error throwing purposes. 
  The new error_if_not_empty parameter is added to the copy_data_between_tables()
  function to indicate the it should return error if the source table isn't empty.