improve the behavior of overwrite mode (mainly for CJK users)

[K. Handa]

I've got a report from a Japanese Emacs user proposing to change the
behavior of self-insert-command in overwrite mode.  Provided that we
have something like the following line (CJK double-width characters),
   あいうえお
point is at 'あ', and we are in overwrite mode, typing "aaaa" results in:
   a a a a お
but typing "\C-uaaaa" results in:
   aaaaうえお

The proposal is to make the former attempt result in the same as the
latter.  I too agree that it is more intuitive.  Attached is the patch
for that.  Shall I install it?