iconv: make utf-7.c able to use variants

Add infrastructure in utf-7.c to handle variants. The approach comes from
iso646.c
The variant is defined at gconv_init time and is passed as a
supplementary variable.

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