Please support rusage argument of waitid

On Linux, the waitid syscall has a fifth argument, a pointer to a `struct rusage`, which works like the corresponding parameter to `wait3`/`wait4`.

Please consider passing an appropriate buffer to this parameter, and putting the result into the opaque `WaitidStatus` struct, with a `cfg(linux_kernel)` function to return the rusage struct.

On `linux_raw` this should be easy (the implementation already has to pass this parameter). On the libc backend, this will require using `syscall!`.