WP 34S and 31S bugs and fixes

[Bit]

The 31S doesn't appear to make use of the quartz crystal (there are no stopwatch or set time functions) but it supports the ON+C key combination, and then it hangs and needs a reset if a crystal isn't installed. I've added a compile time option to disable the crystal completely, and enabled it by default on the 31S.

Someone please let me know in case the 31S does need the crystal in some way I didn't notice.