Migrate all FFI declarations to libc

[Susurrus]

We should remove all FFI declarations within nix in favor of types defined in the libc crate. Much of this work has already been done recently, but there's still some left. And since we don't let any new PRs be merged until libc support has been added, we can resolve this problem relatively easily.

Left to address:

• src/sys/socket/mod.rs (Replace socket FFI with libc versions #746, Fix struct msghdr types on non-Linux platforms. #648)
• src/sys/memfd.rs (Migrate memfd constants to libc #794)
• src/fcntl.rs (see Use upstream libc definitions for fcntl FFI #745)
• src/sched.rs (see Use libc types for sched and syscall FFI #747)
• src/sys/quota.rs (see Migrate quota module to libc FFI types #750)
• src/sys/ptrace.rs (see Moved ptrace constants into enum types plus minor additions #749)
• src/sys/syscall.rs (see Use libc types for sched and syscall FFI #747)
• src/errno.rs (see More libc ffi #798, Fix nix for Dragonfly #684, Switch to libc definitons of errno constants #807)

[Susurrus]

We're currently looking really good for this. The big stopper now is the rest of the FFI types for socket, otherwise I think we're all done.