Auto merge of #3441 - RalfJung:fds, r=RalfJung

shims/unix: split general FD management from FS access

`fd.rs` was a mix of general file descriptor infrastructure and file system access. Split those things up properly.

Also add a `socket.rs` file where support for sockets can go eventually. For now it just contains the socketpair stub.

Author: bors