Upstream and use socket constants from libc

[Susurrus]

Currently src/sys/socket/consts.rs relies on many hard-coded constants. I've recently gone and updated many of these in #636, but there's more work to do here:

• Get all constants that are still numbers upstreamed in libc for all supported nix platforms
• Adapt src/sys/socket/consts.rs to use libc values
• Refactor the constant declarations such that all constants are only declared once with a #[cfg] that only enables the constant on the correct platforms.

To determine what constants are available on what platforms, you can search through the source (see here for a list of sources of various platforms).