[bug-diffutils] bug#30574: Support different charsets

[Victor Porton]

1. We should be able to do `diff` even for source and destination
having different encodings and/or charsets.

Add option for source encoding and destination encoding.

2. The conversion to common encoding should be even with the option to
not fail on wrong or unconvertible characters (like //IGNORE in GNU
iconv), that is replacing unknown characters with a placeholder
character. This is useful to compare wrongly encoded files.

3. More generally we can add filter for every compared file first to
pass through the filter. Item 1 can be implemented by passing `iconv`
or `recode` command as such a filter.