Auto merge of #543 - berkowski:socket_constants, r=alexcrichton

Adds many socket related constants

Started off adding a few constants to help cleaning up some `nix-rust/nix` issues.  Went a little.. further.

Adds lots of socket-related constants (`PF_*`, `MSG_*`, `AF_*`, `SO_*`, `SOL_*`, etc.) to linux/bsd flavors.

Added constants to `bsd/frebsdlike/dragonfly` for completeness, though they aren't covered by CI.  Did not touch `notbsd/linux/s390x.rs`