(ido-unc-hosts-cache): New defvar.

(ido-unc-hosts-cache): New defvar.
(ido-unc-hosts): If value of defcustom is a function, call it to
get list of UNC hosts.  Add function-item choices to specify
ido-unc-hosts-net-view or user function.
(ido-ignore-unc-host-regexps): New defcustom.
(ido-unc-hosts-net-view, ido-unc-hosts): New functions.
(ido-is-unc-root, ido-is-unc-host, ido-file-name-all-completions)
(ido-exhibit): Call ido-unc-hosts to get list of UNC hosts.