Fix error in recent define-option-interface "simplification".

Fix source breakpoints to cope with port whose filename is #f.
Ensure that position recording is enabled when source breakpoint module loaded.

ice-9/debugger/breakpoints/source.scm

@@ -257,9 +257,15 @@
 				       (set-source-property! x' 'column
 							     column)
 				       x'))))
+			;; Don't allow breakpointed expression to have
+			;; a filename property that isn't a string.
+			(or (string? filename)
+			    (set-source-property! x' 'filename "<unnamed port>"))
 			(break! x x')
 			x'))))
 
+(read-enable 'positions)
+
 ; (define g (make-guardian))
 ; (read-hash-extend #\g
 ; 		  (lambda (c port)