Converted `pid` in command executor to `Option<pid_t>`.
Converted `pid` in command executor to `Option<pid_t>`.

This avoids unix's assumption that `pid_t` is a signed integer (which is
not the case on Windows) and explicitly shows whether a pid has been
assigned from launching.

We also changed argument name in `reaped` method to `_pid` to avoid
shadowing.

Review: https://reviews.apache.org/r/66481/

abudnik/mesos,andschwa/mesos,andschwa/mesos,Gilbert88/mesos,asamerh4/mesos,jpeach/mesos,Gilbert88/mesos,chhsia0/mesos,kaysoky/mesos,gsantovena/mesos,reneploetz/mesos,dforsyth/mesos,andschwa/mesos,dforsyth/mesos,kaysoky/mesos,Gilbert88/mesos,jpeach/mesos,asamerh4/mesos,dforsyth/mesos,asamerh4/mesos,gsantovena/mesos,chhsia0/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,kaysoky/mesos,abudnik/mesos,reneploetz/mesos,Gilbert88/mesos,Gilbert88/mesos,gsantovena/mesos,kaysoky/mesos,chhsia0/mesos,jpeach/mesos,asamerh4/mesos,dforsyth/mesos,reneploetz/mesos,chhsia0/mesos,dforsyth/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,andschwa/mesos,gsantovena/mesos,jpeach/mesos,jpeach/mesos,jpeach/mesos,gsantovena/mesos,dforsyth/mesos,chhsia0/mesos,andschwa/mesos,abudnik/mesos,gsantovena/mesos,andschwa/mesos,gsantovena/mesos,kaysoky/mesos,reneploetz/mesos,asamerh4/mesos,kaysoky/mesos,andschwa/mesos,reneploetz/mesos,asamerh4/mesos,abudnik/mesos,jpeach/mesos,asamerh4/mesos,reneploetz/mesos,chhsia0/mesos,Gilbert88/mesos