(ido-mode): Remove ido-ignore-unc-host-regexps from the :set-after list.

(ido-mode): Remove ido-ignore-unc-host-regexps from the :set-after list.
(ido-downcase-unc-hosts): New user option.  Default on.
(ido-ignore-unc-host-regexps): Don't reset ido-unc-hosts-cache
when it is set, as regexps are now applied on the fly.
(ido-unc-hosts): Keep all known hosts in ido-unc-hosts-cache.
Make C-a DTRT--filter hosts through ido-ignore-unc-host-regexps
on the fly, but only when ido-process-ignore-lists is set.
Do case insensitive filtering if ido-downcase-unc-hosts is set.
Only downcase names if ido-downcase-unc-hosts is set.