is broken (prepared statements)": fixed date handling in many places 
of prepared statements code.


libmysql/libmysql.c:
  Fix for Bug#4026:
  - now buffer_length is defined for any buffer type. Network buffer 
    preallocation cleaned up.
  - added constants for maximum buffer sizes necessary for MYSQL_TYPE_DATE,
    MYSQL_TYPE_TIME, MYSQL_TYPE_DATETIME types.
  - TIME/DATETIME packing/unpacking functions fixed 
  - now result set metadata is always updated from fields sent to COM_EXECUTE.
    This is necessary to make 'SELECT ?' queries work without conversions.
sql/item.cc:
  - added implementatoin of Item_param::get_date
sql/item.h:
  - added enum_field_types Item_param::param_type. First step for proper
    handling of placeholders.
  - added get_date() implementation to prevent date -> string -> date 
    conversions when MYSQL_TYPE_DATE/DATETIME parameter is used in temporal 
    context.
sql/protocol.cc:
  Fix for Bug#4026:
  - PACKET_BUFFET_EXTRA_ALLOC -> PACKET_BUFFER_EXTRA_ALLOC.
    The define itself was moved to .cc as it's used only in protocol.cc
  - fixed Protocol_prep::store_time() call.
sql/protocol.h:
  - PACKET_BUFFER_EXTRA_ALLOC moved to protocol.cc
sql/sql_prepare.cc:
  Fix for Bug#4026:
  - MYSQL_TYPE_TIME/DATETIME handling fixed.
  - added initialization for Item_param::param_type in 
    setup_one_conversion_function
tests/client_test.c:
  Test case for Bug#4026