Add an "after fork, before exec" callback

Add an "after fork, before exec" callback

Summary:
In rare cases, it is required to run code **in the child process**, before it calls `exec()` because you need to change the state of the process **before** the child binary gets to run. This diff adds a callback to permit this. The callback is deliberately harder to use than an `std::function`, because you **REALLY HAVE TO KNOW WHAT YOU ARE DOING** to use this. And you have to check your work three times. Even glog `LOG()` must not be called from inside this callback. Your random library of choice is also probably unsafe.

This diff is primarily applicable to job managers and shells. For example, these tasks benefit from this callback: adding a child to a Linux `cgroup`, twiddling its various POSIX process attributes.

Implementing a correct callback for the post-vfork environment is fraught with peril. Read http://ewontfix.com/7/, and the docstrings in this diff.

Reviewed By: yfeldblum

Differential Revision: D2688323

fb-gh-sync-id: aae49e2b3957ca845895acca26e9cb44df1afc07