Add zsh+euc, Zsh with EUC encoding support.

This is just an experimental hack and cannot happily be merged into
the upstream. Zsh's line editor apparently needs a rewrite in order
to support multi-byte encodings because it strongly relies on the
single-byte character scheme.

These patches are mostly based on the work by ono@ono.org (Thanks!):

http://www.ono.org/software/zsh-euc/

What I did over this is disable the hack for non-EUC locales. Maybe
the patches can be moved to shells/zsh in the future, but it's
premature for the moment.

Notes:

• forward-char, backward-char and backward-delete-char with no numeric argument should work properly with this hack.
• Completion and redisplay should work fine.
• There can be some trivial side-effects.
• JIS X0201-Roman and JIS X0208-Kanji are supported.
• JIS X0201-Katakana and JIS X0212 Kanji are NOT supported.
• Only tested with the EUC-JP (ja_JP.eucJP) locale. I'm not sure if it works for GB 2312/CNS 11643-1/KS X 1001. Any feedbacks is welcome, especially a patch if it does not work. :)