WP 34S and 31S bugs and fixes

[Bit]

If an error occurred on the 34S after an operation that didn't disable stack lift, and tracing was also enabled, the stack would be lifted twice, duplicating the X register.

Here's how to reproduce the problem in single precision mode with flag D cleared:
[EEX] [1] [5] [0] [ENTER] [*] [EEX] [9] [9] [*]
Simpler way that also works in double precision mode:
[1] [ENTER] [+] [0] [/]

I've committed a fix. The problem was that xeq() saved the contents of the original stack after it called print_trace(), which in turn calls process_cmdline() and possibly lifts the stack.