(Qw32_charset_hangeul): Rename to match w32 headers.

(Qw32_charset_vietnamese): New symbol.
(xlfd_charset_of_font): New function.
(w32_load_system_font): Use it.
(x_to_w32_charset): Use Fassoc to find charset info. Special case
when Vw32_charset_info_alist is nil to ensure default face always
has font. Use Fcar and Fcdr for safety.
(w32_to_x_charset): Use Vw32_charset_info_alist for mappings.
(w32_codepage_for_font): Use xlfd_charset_of_font.  Use new
pseudo-codepages for special cases.
(w32_to_x_font): New parameter to allow charset portion to be
specified where there is many to one mapping.  Callers changed.
(w32_list_fonts): Avoid listing fonts that won't display.