(map_over_uint8_byte_table): Set Qunbound if an element is Qunloaded.

(map_over_uint8_byte_table): Set Qunbound if an element is Qunloaded.
(map_over_uint16_byte_table): Likewise.
(map_over_byte_table): Likewise.
(map_char_table): Likewise for CHARTAB_RANGE_ALL in UTF-2000; when
CHARTAB_RANGE_CHARSET is specified in UTF-2000 with external database
support, load encoding-table of the specified coded-charset if it is
not loaded yet.
(save_uint8_byte_table): New function of UTF-2000 with external
database support.
(save_uint16_byte_table): Likewise.
(save_byte_table): Likewise.
(Fput_char_attribute): Don't store value into external database even
if the external database feature is supported in UTF-2000; set
`attribute' as name of char-table if the external database feature is
supported.
(Fsave_char_attribute_table): New function in UTF-2000.
(syms_of_chartab): Add new builtin function
`save-char-attribute-table' in UTF-2000.