libedit: improve multibyte support

Until this commit libedit only supported UTF-8 for multibyte charset
Improve it to support other multibyte charsets

Tested with eucJP and SJIS charsets.
Note that this change as been review and committed in upstream libedit
as well via christos@NetBSD

Submitted by: naito.yuichiro _at_ gmail.com
Reviewed by: bapt, pfg, yuripv, 0mp
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D17903