bug#11360: ERC completion case-sensitive

[Stefan Monnier]

> erc-completion-ignore-case or completion-ignore-case is t. A specific
> setting seems like a good idea because presumably some people will want
> case-insensitive completion in erc but not elsewhere.

There are 2 notions of "case-sensitivity" when it comes to completion:
- whether the case is semantically significant in the end result
  (i.e. is "FirstName" another user than "firstname"?).
- whether the user wants "FirstName" be considered as a valid candidate
  for completion of "fir".

The current completion code somewhat conflates the two, but in terms of
"what should Emacs do" the difference is important.  Of course, the
second consideration only applies when the first is true.

So: is the case semantically significant, here?


        Stefan