In testing variable fw/precision in PR_Sprintf, I got a nasty reminder
of the limitations of the current progs ABI: passing @args to another QC
function does not work because the args list gets trampled but the
called function's locals. Thus, the need for a va_copy. It's not quite
the same as C's as it returns the destination args instead of copying
like memcpy, but it does copy the list from the source args to a
temporary buffer that is freed when the calling function returns.
