iswitchb-read-buffer: check that iswitchb-exit is not 'usefirst (set

iswitchb-read-buffer: check that iswitchb-exit is not 'usefirst (set
when user presses RETURN to select buffer) before running:

	;; This happens for example if the buffer was chosen with the mouse.
	(setq iswitchb-matches (list iswitchb-final-text)))

iswitchb-exit-minibuffer: set iswitchb-exit to 'usefirst.

These changes were made to fix the following bug reported by Markus
Rost and John Wiegley:

Evaluate:
(progn
   (iswitchb-mode 1)
   (get-buffer-create "12")
   (get-buffer-create "1")
   (iswitchb-buffer))

You get prompted in the minibuffer.  Type "1".  The minibuffer shows

 iswitch 1{12,1}

Hit RET.  You find yourself in buffer "1" instead of "12", as you
would expect from the documentation.

This bug was introduced when iswitchb-read-buffer was updated to allow
user to select items from the *Completions* buffer with the mouse.