Don't record scratch-fudged registers at an interrupted syscall, because they won't be replayed.

Resolves #329.  May require another round of whack-a-mole with syscalls restarted after rr declines to deliver a signal, since the [semantics](https://github.com/cgjones/rr/wiki/Linux-signals#restart-semantics) are different than for SA_RESTART/SIG_IGN restarts.  But one step at a time.