bug#16577: [External] : bug#16577: Add regexp-based version of completion-ignored-extensions

[Drew Adams]

> It's not great, indeed.  But, AFAICT something like that is the only way
> to introduce regexps into `completion-ignored-extensions`

Please don't introduce regexps into
`completion-ignored-extensions'.  This option
is designed to match file extensions literally.
And that's how it's used.  If you alternatively,
or also, need regexp matching, then add another
variable for that.

Lars claimed this:

   completion-ignored-extensions is used many
   places (and it's commonly used as a regexp)

No evidence given for that claim.  I asked for
examples of such "common" use.  None have been
cited.  Are there even _any_?