(Vcharset_latin_tcvn5712): New variable.

(Fput_char_attribute): Set up `Vcharacter_variant_table' instead of
`Vcharacter_composition_table' if `->decomposition' property has only
1 element.
(Qlatin_tcvn5712): New variable.
(syms_of_mule_charset): Add new symbol `latin-tcvn5712'.
(complex_vars_of_mule_charset): Add new coded-charset
`latin-tcvn5712'.