unknown_8bit stuff don't work in real.

[Kang Jeong-Hee <Keizi mail co kr>]

after cvs up -dPC, ./autogen.sh again; make; make install
but setting pref for unknown_8bit work well for save and restore.
~/.gnome2/balsa remember them.

but on opening a mailbox, first occurance of libbalsa_utf8_sanitize()
in part_info_init_mimetext() get args of fallback=false, codeset=0.
and never parse 'unknown' string as preference.

it's out of expect.
I don't know why.

kz