read-file-name argument clash with xemacs

[Noah Friedman]

XEmacs's 6th argument to read-file-name is a history list variable.
In the Emacs development sources, it's a predicate.

This is a somewhat nasty situation because functions which currently
inspect the argument list or use a condition-case to trap
wrong-number-of-arguments, are suddenly failing.

So I suggest adding the history variable support to Emacs and moving the
predicate out to the 7th argument.  Any objections?