Add some constants to libc

[kamalmarhubi]

NB This issue describes making changes to the libc crate, not to nix. The issue is here because these are constants we'd like to use from libc as part of work on #264, but are not yet available in the libc crate.

I've been going through bitflags as part of #315, and taking note of some places where we need to get constants upstreamed.

• sys::memfd::MFD_* (Add constants for memfd_create syscall rust-lang/libc#836)
• sys::socket::consts::MSG_*
• sys:socket::consts::SOL_* (linux: add missing SOL_* constants rust-lang/libc#228)

[posborne]

I have a PR open for sys:socket::consts::SOL_*: rust-lang/libc#228 although those are not flags.

[kamalmarhubi]

Definitely add any non-flags up above too. I'm just looking at flags quite closely because of #315.