Please support using wait4 for Child and Command

[joshtriplett]

On Linux, the wait4 system call provides all the same information as waitpid, but additionally returns a struct rusage containing the resource usage of the child process. I'd love to have Child and Command provide a platform-specific extension trait that uses wait4, and then returns the struct rusage in addition to the exit status.

[cuviper]

It would be nice to just add methods to ExitStatusExt for ExitStatus, as this is where we use waitpid today, but currently that type is just a c_int wrapper, so adding struct rusage would be a lot.

So I guess you're suggesting to provide alternatives for Command::status and Child::{wait, try_wait}, and maybe some alternatives for the methods returning Output too?

[joshtriplett]

Replacements for the whole range of functions seem like overkill; it'd suffice to have some way to call wait4 on a child (returning an rusage and a status) and then separately obtain the output if desired and captured.