Support linux namespace clone flags

Support linux namespace clone flags

Summary:
This diffs adds supports on folly::Subprocess to be able to take clone flags and
use them to call clone(2) instead of the default vfork()

I checked that all tests pass when I replace vfork with fork on trunk. So there
isn't anything built assuming the parent is paused for an execve. Correct me if
I am wrong here.

(Note: this ignores all push blocking failures!)

Reviewed By: snarkmaster

Differential Revision: D4853169

fbshipit-source-id: 7e5851df3a49996a4a5dc1945457686dd042e1f4