MOSDDBLK: always reserve 2 drives for floppies [fixes roelandjansen#21]

This is a quick-fix that prevents HDD from moving to A:.
It is not clear why HDD does not work on A:, maybe it should?
Maybe someone else can code up the more intrusive fix.

SOURCES/src/kernel/MOSDDBLK.ASM

@@ -2560,7 +2560,7 @@ finitblk proc near
 fini10:
 	mov	bx,[hdr]
 	inc	al			; maximum drive number --> # of units
-	mov	es:[bx].irUnits,al	; set # of units
+	mov	es:[bx].irUnits,2	; set # of units
 	mov	al,es:[bx].irDriveNumber ; starting drive number
 	inc	al
 	mov	[startd],al