Grabbing file descriptors with pidfd_getfd()

Using sendmsg(2) requires co-operation from the other side (or the injection of parasitic code a-la CRIU or rr). Those approaches are really suboptimal for a bunch of reasons, and having an interface which does this properly and doesn't require shellcode injection as part of normal code execution is a massive benefit. Not to mention that seccomp filters on the target process may block some of the syscalls needed for that to work.