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

[Stefan Monnier]

> That is, completion-ignored-extensions as before, and
> completion-ignored-files (a list of strings) and
> completion-ignored-regexp (a regexp or a list of regexps).  The
> advantage is that the semantics on all three variables would be pretty
> self evident, and would require no special syntax anywhere.

[ Not sure I'd bother with the non-regexp variable (other than having to
  use \\. to match a dot, a regexp should work just as well).  ]

This is the best way long term, yes.  We could mark
`completion-ignored-extensions` as obsolete and in a few years we're
back with a clean semantics and a simple variable.


        Stefan