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

[Drew Adams]

> This should make the change much less invasive, with most existing uses
> of `completion-ignored-files` unaffected.

Most existing uses where?

If you change the meaning/behavior of
`completion-ignored-files' you'll undoubtedly
break existing code somewhere.  Till now, the
extensions have been literal extensions.

If something else is needed to handle directory
names (and not even the extensions of dir names
that have extensions), then add/invent that.

Why change the use/meaning of
`completion-ignored-extensions?