iconv: Better mapping to RFC for UTF-7

- Direct use of characters instead of arcane arrays
- isxbase64 is not the Modified BASE64 alphabet, but the characters who
  needs to trigger an explicit shift back to US-ASCII. Make that clearer

Signed-off-by: Max Gautier <mg@max.gautier.name>
Reviewed-by: Adhemerval Zanellla <adhemerval.zanella@linaro.org>