RE: buffer name completion is case-sensitive now

[Drew Adams]

> But since the buffer system is not really tied into the file system, I
> think it a reasonable hypothesis that the completion default 
> should not depend on the file system completion default.

I agree.

> I guess we already have Drew's vote about buffer name case sensitivity
> on Windows in (even though he confuses "Windows" and "standard").

I think we should add a user option for this, similar to
`read-file-name-completion-ignore-case': `buffer-name-completion-ignore-case'.

I don't care much what the default value of that option would be. I'd vote for
t, but the important thing is to give users a way to get the behavior they want.