(Valternate_charset_table): The valiable deleted.

(Valternate_charset_table): The valiable deleted.
(Venable_character_unification,
Vstandard_character_unification_table_for_read,
Vstandard_character_unification_table_for_write,
Qcharacter_unification_table): New variables.
(syms_of_coding): Initialize and declare them.
(DECODE_ISO_CHARACTER): Modified to handle a character unification
table instead of Valternate_charset_table.
(DECODE_DESIGNATION): Delete handling of Valternate_charset_table.
(decode_coding_iso2022): Handle a character unification table.
(ENCODE_ISO_CHARACTER): New macro.
(encode_designation_at_bol): Handle a character unification table.
Do not return -1 even if end-of-line is not in the current run.
(encode_coding_iso2022): Handle a character unification table.
Call macro ENCODE_ISO_CHARACTER instead of calling
ENCODE_ISO_CHARACTER_DIMENSION1 and
ENCODE_ISO_CHARACTER_DIMENSION2 directly.  Check the size of
carryover before copying it to destination.
(setup_coding_system): Initialize the member
character_unification_table of the struct coding system to Qnil.
(Fset_keyboard_coding_system): Doc string augmented.