The lib used to return "" for both properly detected ASCII and
detection failure. And the tool would return "ascii/unknown".
Make a proper distinction between the 2 cases.
It was not clear if our naming followed any kind of rules. In particular,
iconv is a widely used encoding conversion API. We will follow its
naming.
At least 1 returned name was found invalid: x-euc-tw instead of EUC-TW.
Other names have been uppercased to follow naming from `iconv --list`
though iconv is mostly case-insensitive so it should not have been a
problem. "Just in case".
Prober names can still have free naming (only used for output display
apparently).
Finally HZ-GB-2312 is absent from my iconv list, but I can still see
this encoding in libiconv master code with this name. So I will
consider it valid.
