curses: add option to specify VGA font encoding

This uses iconv to convert glyphs from the specified VGA font encoding to
unicode, and makes use of cchar_t instead of chtype when using ncursesw,
which allows to store all wide char as well as the WACS values. The default
charset is made CP437 since that is the charset of the hardware default VGA
font. This also makes the curses backend set the LC_CTYPE locale to "" to
allow curses to emit wide characters.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: Eddie Kohler <ekohler@gmail.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190311135127.2229-3-samuel.thibault@ens-lyon.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>