apple2e: Need ability to disable No-Slot Clock

[inexorabletash]

The DS1315 NSC is a "required" device in apple2e; the ability to turn this off via a flag would be nice e.g. for validating the clock drivers and apps do the right thing when the clock is not present.

When the clock is present, it would also be nice to be able to set the clock from outside the emulated machine, either on startup or at runtime via Lua scripting. From within emulation the clock can be set using bit-banging but that's not always convenient.

[inexorabletash]

cc: @arekkusu42

[rb6502]

I think the way to go with this is going to be a config switch. Working on it.

[arekkusu42]

Maybe apple2p should also accept this new switch? There are NSCs compatible with 24-pin ROMs for ][ and ][+.