impossible fix - potentially invalid feedback

impossible fix - potentially invalid feedback

     From a bug report, the message displayed to the screen by
impossible() might be different from the one written into paniclog, if it
had argument subsitution/formatting.  I couldn't reproduce that myself,
but stdarg.h/varargs.h is tricky stuff and I think that passing the va_list
to a routine which steps through it requires that va_start be called again
if you're going to use the va_list a second time.  This changes impossible()
to handle its arguments only once, like panic().