(ido-cache-ftp-work-directory-time): Doc fix.

(ido-cache-ftp-work-directory-time): Doc fix.
(ido-unc-hosts): New user option to explicitly define list of know
UNC-style hosts for completion.
(ido-cache-unc-host-shares-time): New user option.
(ido-is-unc-root, ido-is-unc-host, ido-cache-unc-valid): New
helper functions for UNC file-name support.
(ido-may-cache-directory): Check for UNC host.  Simplify.
(ido-wash-history): Clean out old UNC hosts.
(ido-nonreadable-directory-p): UNC hosts are always readable.
(ido-directory-too-big-p): UNC hosts are never too big.
(ido-set-current-directory): Handle UNC root path.
(ido-file-name-all-completions): Complete UNC host names from
ido-unc-hosts list.  Cache UNC host shares.
(ido-make-file-list-1): Don't filter UNC root.
(ido-exhibit): Check for // in root directory, and switch to UNC
mode by setting ido-current-directory to //.