(Qvietnamese_viscii_lower): New variable.

(Qvietnamese_viscii_upper~: New variable.
(Fdefine_charset_alias): New function.
(syms_of_mule_charset): Add new function `define-charset-alias'.
(syms_of_mule_charset): Rename charset `vietnamese-viscii-*' to
`latin-viscii-*'; define `vietnamese-viscii-*' as aliases for
`latin-viscii-*'.