std::process should provide a facility to specify custom file descriptors to stdin/stdout/stderr #893

Currently, RFC #517 specifies that "The equivalent of today's InheritFd will be added at a later point.". However, this severely limits the utility of std::process. My current use case is writing a (toy) shell, without resorting to libc::*. Unfortunately, without support for setting custom file descriptors in std::process, it's not possible to implement features like file redirection or pipelines.

From my reading of libstd/process.rs and libstd/sys/${PLATFORM}/process2.rs, it looks like all of the platform-specific code for handling AnonPipe/FileDesc is already in place, and a small change to the API of process.rs would allow for setting custom AnonPipe.

I'm happy to work on a patch (assuming that I haven't missed any important details in my analysis regarding difficulty).