small improvements for LOAD command and IEC routines

c64/basic/set_sane_devnum.s

@@ -0,0 +1,17 @@
+
+set_sane_devnum:
+
+	;; Take last device number, make sure it's a drive
+	;; If not, set to 8 (first drive number)
+	;; Device number is left in X, for SETFLS Kernal routine
+
+	;; Note: if tape support is introduced here, make sure DOS wedge won't get broken
+
+	ldx current_device_number
+	cpx #$07
+	bpl +
+	bcs +
+	ldx #$08
+	stx current_device_number
+*
+	rts