C3S: Add (another) switch to disconnect the !SD line

[andrewgreenberg]

Making the !SD line work with the external supply works too well - when you program the actual C3, the entire bus goes down and kills the programming which kills the C3 which then times out...

So, add another switch which disconnects !SD from the !SD disconnect transistors so the external power supply can always be left on, whatever the state of the !SD line.