Make non-ISO-2022 CJK reencoding work again

Fix https://github.com/abelcheung/fontforge/issues/2 , which was there
since 2007 or probably even earlier. It is likely caused by overzealous
change replacement of unichar_t which broke double byte char detection.
Drop ISO-2022-CN encoding, equivalent to EUC-CN

There is no reason to retain 2 equivalent encoding (where one is
transformed to another via adding or subtracting fixed value), and
iso-2022 one is rarely used
Drop old encoding list for old SFD versions
Cosmetics for CJK encoding names in UI
Rename "gb2312pk" enc name, which is really GB2312
Merge branch 'reencode-cjk-fix'