xrun: behave reasonably in the face of an unlimited stack size rlimit (thanks Make)
xrun: behave reasonably in the face of an unlimited stack size rlimit (thanks Make)

GNU Make helpfully increases the stack-size soft rlimit for us:

```
bpowers@codemonkey:~/src/DoubleTake$ cat /proc/self/limits  | grep stack
Max stack size            8388608              unlimited            bytes
bpowers@codemonkey:~/src/DoubleTake$ make rlimit V=1  | egrep 'cat|stack'
cat /proc/self/limits
Max stack size            unlimited            unlimited            bytes
```

This caused a cascading bunch of problems.

plasma-umass/DoubleTake,bpowers/DoubleTake,bpowers/DoubleTake,bpowers/DoubleTake,bpowers/DoubleTake,bpowers/DoubleTake,plasma-umass/DoubleTake,bpowers/DoubleTake,plasma-umass/DoubleTake,bpowers/DoubleTake,plasma-umass/DoubleTake,plasma-umass/DoubleTake,bpowers/DoubleTake,plasma-umass/DoubleTake,plasma-umass/DoubleTake