Subprocesses are not terminated when the parent process exits

[jspahrsummers]

Originally reported and discussed in Carthage/Carthage#89. The most notable comment is this:

From a lot of Googling and consulting with @keithduncan, it appears that NSTask is a special 💩❄️ in this scenario, and spawns processes outside of the parent's process group. The same problem supposedly wouldn't arise with fork() or posix_spawn() directly.

[ikesyo]

Using the NSConcreteTask.setStartsNewProcessGroup private API would be the solution, which is used in various projects:

• https://github.com/facebook/xctool/blob/26a20314dcef4c238012889fbe37b767ec5719ca/Common/TaskUtil.m#L440-L459
• https://github.com/CrossWaterBridge/xctool-runner/blob/b2f0aac94a8c19f033e3f39df4c2e718403b5aab/xctool-runner.swift#L20-L34
• https://chromium.googlesource.com/chromium/src/+/4884682f47e2c9f0c5b193e6d531714821dd8ffc/testing/iossim/iossim.mm#69