In server we assume that datetime values stored in MYSQL_TIME struct
are normalized (and year is not greater than 9999), so we should 
perform range checks in all places then we convert something to
MYSQL_TIME. 


include/my_time.h:
  Added one more argument to set_zero_time() function to make it more 
  convinient.
  Added comment clarifying why MAX_DATE_STRING_REP_LENGTH value is 30.
include/mysql_time.h:
  Documented MySQL's internal assumptions for members of MYSQL_TIME
  structure.
libmysql/libmysql.c:
  It does not make sense to set MYSQL_TIME::time_type twice in case of 
  errors.
mysql-test/r/type_datetime.result:
  Added test for bug #6266 "Invalid DATETIME value not handled properly".
mysql-test/t/type_datetime.test:
  Added test for bug #6266 "Invalid DATETIME value not handled properly".
sql-common/my_time.c:
  str_to_datetime(): Added missing check for too big year values.
  set_zero_time(): added time_type argument, since MYSQL_TIMESTAMP_NONE
    is not the value that we want in most cases.
sql/field.cc:
  Field_datetime::store_time():
    clarified why we don't perform any range checks here.
sql/item.cc:
  Item_param::set_time():
   Added comment describing this method and range checking for TIME
   values.
sql/sql_prepare.cc:
  Removed comments about range checking for TIME values in prepared 
  statements, which are no longer true.
  set_zero_time() has one more argument now.
tests/client_test.c:
  Added test for bug #6266 "Invalid DATETIME value not handled properly"