Allow prefix arg for ido-find-*file* (maybe)

[Sean O'Rourke]

ido mode is currently unusably slow when finding a file in a large
directory.  For example, in a directory with ~3000 files,
`ido-find-file' takes several seconds to come up with a prompt.  When
you know you're in such a directory, it would be nice to bypass ido
entirely and fall back to the original `find-file'.  Since the ido
functions currently don't use a prefix argument, I think that when
called with a prefix argument, the ido functions should fall back to
their non-ido equivalents.

/s

ps -- here's a defadvice to get the effect I want:

    (defadvice ido-find-file (around allow-prefix compile activate)
      (if current-prefix-arg
          (let ((read-file-name-function nil))
            (call-interactively 'find-file))
          ad-do-it))
    
    (defadvice ido-find-file-other-window (around allow-prefix compile activate)
      (if current-prefix-arg
          (let ((read-file-name-function nil))
            (call-interactively 'find-file-other-window))
          ad-do-it))
    
    (defadvice ido-find-alternate-file (around allow-prefix compile activate)
      (if current-prefix-arg
          (let ((read-file-name-function nil))
            (call-interactively 'find-alternate-file))
          ad-do-it))