Audit all cmsg code

[asomers]

control messages used by sendmsg and recvmsg are one of the hardest Unix APIs. Some of Nix's cmsg-related tests currently fail on OpenBSD (see #1001), and others rely on a hard-coded os-dependent value that is not defined anywhere in libc (align_of_cmsg_data). Both of these things hint at underlying bugs in our cmsg implementation. We should audit the entire thing for correctness, and possibly rewrite it to be more C-like. libc already exports versions of the CMSG_DATA family of macros. Nix should use those rather than reinvent their logic.

[asomers]

Blocked by rust-lang/libc#1210

[asomers]

Ok, I've got this working now. It passes the tests on FreeBSD, OpenBSD, and NetBSD. But now the scm_credentials tests fail on Linux. I suppose I'll keep working on it.